If my understanding is correct, the levels don't really get any better over 100. They just sort of cap themselves at 100 no matter what they are. I may be wrong though.
Virtually all attributes over 100 have benefits:
- Strength, increases the encumbrance limit to let you carry more. Increases "fatigue" aka stamina. But no additional weapon damage.
- Intelligence, increases magicka
- Endurance, increases health and "fatigue"
- Willpower, increases magicka regen (except for an Atronach), increases "fatigue"
- Agility, increases "fatigue". Possible reduced stagger rate, not sure if this has been proven. Possible improvement in sneaking but I'm not convinced. No increase in bow damage
- Speed, you will move (running and swimming) faster
- Personality, probably a higher disposition rating, but not really significant
- Luck, better results with a few things like arena betting and Mehrunes Razor.
Skills generally show no improvement over 100, except for acrobatics and athletics.