Even if you're not a strength athlete and your only goal is building muscle, making strength training one of the foundations of your exercise plan can provide enormous benefits. If you neglect strength work, you are leaving a ton of potential progress on the table and setting yourself up for plateaus. In order to make continual progress without stalling, you need both strength and muscle hypertrophy to take place, as each assists the other and both are forms of progressive overload.
Progressive overload is key!
So why is strength important to someone who only cares about building muscle? The simple answer is because of progressive overload. Progressive overload is king when it comes to making progress. If you are adding weight to the bar, you are progressively overloading your muscles and giving your body a reason to improve and grow. If you are capable of doing 5 sets of 5 with 200 pounds on the bench press, and then take those 5 sets of 5 to 250 pounds, your body has no choice but to adapt and grow because of the new, harder stimulus placed upon it.
There are multiple ways to cause progressive overload. In my article Understanding Volume & Fatigue, I talk about increasing training volume by increasing the total amount of sets you do each week. This is a form of progressive overload as well, and while I do mention in that article that tracking and increasing your total sets is a great way to build muscle and increase training volume, there is a limit to how many sets you can do per week. Past this point, gaining strength becomes vital to continue progressively overloading your body, because you're no longer able to add more sets to your workouts without impeding recovery. Without utilizing heavy strength focused sets, you will potentially end up spinning your wheels by not training heavy enough to elicit strength increases, so when you reach a point where you need to add weight to the bar rather than sets, you'll stall until you begin to train heavier.
An example would be if I were doing 10 sets of bench press per week and I gradually increased it to 25 sets per week, I’d make great progress because I increased volume, but then what? I can’t simply continue to add sets and end up doing 30+ sets per week. Not only is that impractical, but it would be unsustainable and either lead to injury or over training. There are only so many sets you can do and still be able to recover properly. This is where progressive overload in the form of strength comes into play. By training heavier and increasing your strength levels, you can now handle heavier loads at the same amount of total sets, thus increasing your total volume in terms of tonnage (Sets x Reps x Weight). Remember, both adding sets and adding weight to the bar are ways to increase total volume and are both types of progressive overload.
In order to elicit continual progress, you need to utilize both methods of progressive overload. For example, you can begin by increasing total sets done and once you reach a point where adding more sets becomes impossible without recovery issues, begin to focus on strength gains. Once strength gains are made, you could even reduce the total amount of sets you're doing while still providing your body with a stronger overall stimulus to grow, due to working with heavier loads. Then, you could begin to add sets slowly until you hit your volume limits again and then switch your focus to strength once more.
This process can be repeated continuously and is a great way to periodize your training and is one common way to perform what's called linear periodization, where you would start with lower intensity, but higher volume and then slowly through the weeks more towards higher intensities and lower volumes, after which point you take a deload and repeat the process again. This is a tried and true method of training and many follow this process all the way into elite levels of fitness.
Practical Application - Cycle intensity & volume
The smart way to train is to make sure you aren't neglecting high volume training or strength training, but to train them both, at least periodically - hence why periodization exists. Training both is important because volume work assists strength work and vice versa. By training for strength, you increase the loads you can use on your volume days, and by training for volume you build more muscle which in turn will increase strength potential. By only training for strength, you'd typically be forced to train with less total sets in order to facilitate recovery due to heavier lifting being more taxing, which will be detrimental to muscle hypertrophy. By training only for volume, your strength gains will be sub par, so when hitting your volume limits in terms of total sets, progressive overload will be harder to achieve unless you increase strength and add weight to the bar. This is not to say that strength training does not build muscle and volume work does not build strength, but they do so at a vastly slower pace than if you focused on the related training style. By combining the two you have all grounds covered and are able to make consistent and optimal progress.
My favorite way to structure training is to cycle between high volume days and low volume but high intensity (strength) days within the same week. I prefer to train every muscle group at least twice a week, but how you split this up is up to you and as long as training intensity and volume are relatively equal, progress will be the same. Since I like training each muscle group twice a week, each muscle group will have one strength day and one high volume day. My schedule of choice is what is called an upper/lower split, where you have two upper body workouts and two lower body workouts per week. Here is an example schedule:
Monday: Upper Body Strength
Tuesday: Lower Body Strength
Thursday: Upper Body Volume
Friday: Lower Body Volume