How exactly does this work and why wouldn't you set all atts. to 100?

That's the weight each attribute has to calculate each player's composite value (the 'future' and 'current' values for each player). You may disagree with how important certain attributes are, i.e. you may want to emphasise a running game so you are more concerned with run blocking than pass blocking. So if you change your attributes to 50 for pass blocking and 100 for run blocking, when you look at a player's rating you will see the better run blockers having higher current and future ratings, and the better pass blockers will not score as high if they don't have good run blocking skills. This allows you to customize what rating values each player gets to your liking. It has nothing to do with how the player performs, only what the current and future numerical values are for you personally.

But I don't see individual player weights, only positional.

You set the weights for how a position should be evaluated, and from then on, the current and overall scores that you see for any player at said position will be calculated based off of YOUR determined values.

IE You have two players on your team at LT listed as a 75/75 and 68/68. You change your player weights for the LT position to better emphasize a certain attribute, such as run blocking. Now you review your roster to find that the first LT has changed in rating to be a 78/78 and your other has dropped in value to 66/66. If you re-set the values back to their original setting, you'll see those two player's overall value return to what it was.

Thank you