Just like developing time, developer concentration, and developer temperature, agitation is a tool that you can use to affect the results you get.
You can choose to ignore this, and develop the same every time, but it's good to know that how you agitate will change your negatives.

Basically, what happens is this: If you agitate often, you will develop your highlights faster. In the highlights is where your negative received most light exposure, and the developer will exhaust quicker there than in the shadows (which are thin on the negative). When you agitate you replenish with fresh developer across the entire film surface, so there is fresh developer in contact with the highlight areas of your negatives more often. That accelerates the development rate of your highlights in relation to the shadows. Make sense?

Conversely, if you agitate less frequently, say 10 seconds every three minutes, you retard the rate at which the developer develops the highlights in relation to shadows. I hope you see the pattern. There are no rules for how you SHOULD develop your film, just guidelines, which usually produces a technically decent result, and it's a good place to start. All I'm saying is that how you agitate will have an effect on the outcome, and it is you who should decide what your negatives are like since it is you who are going to use them to make prints.