Training frequency is an often overlooked variable that can have quite a bit of impact on your overall progress. Many lifters still use a body part split, and only train each major muscle group one time per week. While this strategy typically works ok for beginners, at some point you're going to need to move on to something more effective.
Increasing training frequency, especially for lagging body parts or lifts, can be a great way to increase both muscle hypertrophy and strength. Let's use bench press strength as an example. If you are looking to improve your bench, training your bench more often is a great way to make progress. Think of it this way--increasing your bench training to 2x per week raises the number of times you bench press from 52 x per year to 104 x per year. That's an extra 52 chances to get better.
Increasing training frequency serves several purposes:
1) Increases the amount of technical practice a lifter gets on each of the competition lifts. An increased level of competition specific frequency is most important when preparing for an upcoming meet.
2) Allows lifters to increase their total training volume for a certain lift and/or muscle group by spreading training volume for a lift or muscle group across multiple days. It also allows lifters to recover between sessions of each lift while still keeping total training volume high. The next post is dedicated specifically to this topic, so more to come on this.
3) Training a muscle group more often typically leads to greater hypertrophy across a set time period. In a 2015 study comparing training frequency while overall training volume was matched, the higher training frequency group had significantly more increase in muscle hypertrophy.
4) As a general rule, increased frequency equals increased strength.
When adjusting frequency, it is important to remember that, like all training variables, the best training frequency for an individual is going to be specific to that individual. Training bench press 4x a week might work great for one lifter and be too much for another. Ultimately, sustainability and adherence are key to progress, so training frequency should be set to a level that allows a lifter to adhere to their training program. Increased frequency can also increase injury risk, therefore increasing frequency should be done slowly.