The main purpose of doing kegels is to increase the strength of your PC muscle, allowing it to pump more blood to your penis in each use and force semen out with more pressure. The additional blood will aid in any girth work and also in the quality of your erections. Kegels have a secondary purpose too, to teach you better control over this muscle and therefore your ejaculation.
So, you're saying that you have perfect erections already and asking if you need to bother with kegels if all you want is to gain size? The answer is yes.
As you gain in size, the volume of your penis increases massively, meaning that the blood that fills you now will be pitiful in the future. Yes, your PC muscle will grow on its own as it tries to fill your new size but it won't happen quickly so your erections will suffer somewhat. Putting in extra effort to grow the muscle now will save you a lot of worry in the future.