I believed a FOV increase was to see more on the sides.
As has been elaborated, FOV = zoom. I think you're confused by the fact that the FOV change that we talk about as part of hor+ widescreen implementation results in no perceived zoom change between 4:3 and widescreen. However, in that case that's because that's because the shape of the viewspace is changing along with the FOV. At the most literal level a hor+ change is zooming out; it's just that because shape of the viewspace also changes there's no perceived zoom change vs 4:3. Changing the FOV while maintaining the same aspect will certainly alter the actual and perceived level of zoom.
So back to topic, one question: Which FOV setting is now optimal for which kind of user? 16:10 (1680*1050 for example) is there the FOV 90 optimal and for 1024*768 the FOV 75?
Note that the FOV setting indicates what the FOV is at a 4:3 ratio which is then adjusted appropriately to maintain hor+ for widescreen aspects.
In other words if you select FOV 75 and run a 16:10 screen your FOV isn't actually 75°, it's whatever the hor+ "equivalent" is (in the case of a 16:10, the actual FOV would be 85° according to the FOV Calc).
So therefore one FOV setting isn't suited to one aspect over another. You're always getting the "equivalent" FOV for your aspect regardless of the the setting. It's purely a matter of whether or not you like a smaller or larger FOV.