Lowers probability of colon cancer. Vitamins and minerals have already been mentioned. There’s also some proteins; generally not much although beans are a proteiny veggie, and for example green beans combine high fiber with high protein.
Excuse me while I move this soapbox into place…
Many people think that having a multivitamin is the same as “eating healthy.” It’s not. They say “oh, but if I just pop my Big Healthy Pill, I don’t need to worry about my veggies, right?” Oh, so, wrong.
First, we do need fiber.
Second, if you’re not eating veggies and fruits you’re likely to be overeating other less healthy stuff. Popping your Big Healthy Pill doesn’t magically make that Big Mac good for you.
Third, the composition labels in many of those multivitamins make me want to scream. Their minerals are water-soluble sats, but often the combinations of two of those salts are insolluble. You have a salt AB (solluble) and another CD (solluble), but once it’s in a glass or in your mouth the ions produced when those two salts dissolve don’t have a label saying “A which goes with B”: if AD is insolluble you’ll be eating AD pebbles :p. Oh, and if any of the salts is a carbonate then yeah, your Big Healthy Pill will make you gassy.
It’s a bit of an exaggeration and one of the interesting properties of the digestive tract is that it has pH extremes, but a lot of times the salts chosen produce several combinations which are insolluble at all pHs. Expensive pebbles.