Steam is a program that acts as a medium for distribution, server finding, etc. It cuts out the most of the intermediates between developers and gamers..
http://en.wikipedia.org/wiki/Steam_(content_delivery)
I don't know about RO, most everyone I've heard says it's really good. Steam has been hit & miss for me. It lost the info on one of my old accounts, and there was no way to get it back.. They were probably just swamped with similar tech support requests and I must've fallen through the cracks. Some people have horror stories, others have never had any problem with it.
I only dislike it cause it seems to eat resources while on "standby" in the tray, but never really did the math. It doesn't seem to leave anything behind when you tell it to uninstall, so I'd say your best bet is to give it a try.
Someone with more experience will comment, but that's my data point for you.
E.G. of a PITA situation:
Payment issues
Due to Steam products' ephemeral nature and Steam's resultant susceptibility to fraud, billing details entered must match exactly with those held by the bank.[62] Similarly, certain types of failed transactions (such as chargebacks) will cause the user’s Steam account to be disabled until audited by support staff.
