Premium fuel, in addition to the higher octane rating, will sometimes also contain additional detergents.
Octane rating is the most important. The higher the rating, the more compression it can withstand before detonating. This is very important in vehicles with higher compression ratios, typically turbo / supercharged & performance engines. These days, a lot of engines' ECU can compensate for inferior octane / fuel, but it is still not good for the engine, and your mileage will also suffer.
Short story, put in what the manual recommends.
