Why agitate the film during developing?

Photography question for a school project

You want to make sure the film is developed evenly, not just the bottom half. I'll let you figure out how to explain it in more detail. I'm not doing all of the work for you.

you should only agitate it for 5 seconds of every minute that you are developing. That is if you are doing B &W film. Color I am not sure

So if the film stick togther on the reel the developer will still cover the spots that are covered and you don't only do it for 5 seconds you do for half the time you are developing (so every other 30 secs) and then you shouldn't only have half of your film submerged in developer is should be fully submerged.

You actually don't have to agitate-many folks use Rodinal in stand development, where the film gets one agitation right after the developer is poured in and then sits for anywhere from thirty minutes to an hour.

This results in a fairly low contrast negative, while agitating increases contrast and dramatically shortens developing time.

Here's why:
The only developer doing that's actually doing the work of developing is that which is in contact with the film. After a developer molecule has reacted with a silver halide, it is used up-it can no longer react with the silver halides.

If the developer is not moving, the molecules won't move around very much. This means that immediately above the areas of higher density, the concentration of unused developer molecules drops off, and the rate of development slows down very dramatically. This means that areas of high density essentially stop developing.

The problem doesn't exist as much in areas of low density, as there aren't nearly as many silver halides reacting as to exhaust the developer. Thus, in areas of low density, developing continues at a greater rate than it does in areas of high density.

Thus, over the course of the development time with no agitation, the shadow areas will get more development than the highlight areas, resulting in lower overall contrast.

Conversely, agitating provides a continual supply of fresh developer to the surface of the film. By agitating once every thirty seconds or so, you are keeping the amount of development essentially the same for all areas of the film.