Using AI levels above 105% may cause it to become too aggressive and unpredictable. They are basically trying to drive too fast for their "skills".. Also some track use "cheat" grip, AI has different griplevels that human drivers.. Mostly it's just a percent or two but this game is accurate enough for even fractions of percent to be noticed... So if the AI drivers are braking 30m later, they most likely have more grip. Also AI is not good at corner exits so you can easily pass the m on straights. With humans, that is most likely the opposite: brake sooner and take much better exit speed (just like we should, it's the fastest way to drive).
In essence, you can do some practice with AI on certain tracks but the main racecraft is found only online. AI lacks the finess of human drivers, it doesn't have feelings and reacts always in the same manner. You lose all the psychological part of the game, that is a major skill to learn... You can drive a human off the track without ever touching him or doing anything illegal or evil.. Just push him out of his limits by sticking to his rear, challenging from every mistake, brake always later and he drives wide in the Parabolica. within two laps...
Human drivers also WON'T slow down if you cross their drivepath just in front of them going the same speed.. With AI, after you pass, chance lanes in front of him at the next corner exit and he slows down. No touching, just make sure your rear bumber is close enough to his front one when accelerating and they brake everytime.. Once you learn that trick, AI is no use for you anymore.. For just laptimes comparison, 105% is usually enough. Varies per track. like said in the posts above.
Time attack on the other hand is useful tool.. Fast way to learn a new track and to do qualify kamikaze lap recon. Not good for setting up the car, only for initial setup. It's "alone" mode, there's just you on track which can be boring sometimes..