MC over-trousers (oh go on over-pants then), will:
1) Add warmth & windproof - mine have a removable thermal liner
2) Add knee and usually hip impact protection in a crash - had to buy the extra hip armor for mine separately, but they already had the special pockets for the armor to fit into
3) Resist any scraping along the ground - reduction in road rash
4) Keep your trousers clean & looking smart
5) KEEP YOUR LEGS DRY when it rains - thats a big thing, trust me
Personally, I won't ride without them, whatever the weather, but that choice varies for each person