Since decoction requires the malt to be boiled, you can get a whole lot more of the starches very well gelatinized -- basically making a starch soup -- which then when added back into the "main mash" (which is the liquid portion), the enzymes can take action more readily, which can improve your efficiency by quite a bit. On my decoctions I've seen a jump in efficiency of 5-10%.
Other than that, yeah, you can get darker color if you boil the decoction for a long time. But this isn't necessary either. I usually only boil for like 10 minutes which gives you all the benefits without extending the brew day by much, and not darkening as much either.
Overall....... while I've tried decoction and I understand its uses... I don't do it anymore. It's sort of fun to experiment with, but it's just really not necessary with the well modified malts we have in the 21st century. Decoction was more useful hundreds of years ago when malt quality was not as great. Now if you go shopping for undermodified malts, you just really can't find it anywhere. I know -- I've looked, many times. Fact: Any so-called "undermodified" malts sold today actually aren't.