I see a huge amount of people start out with RoA on their casters and recommend others do the same, and I don't really get why. Considering that mana is virtually meaningless on Dominion, with a few notable exceptions, it's basically 3035 gold for 630 health and 80 ability power, and a passive which, imo, also doesn't do much in Dominion. For 70 gold more, you can get Rylai's with gives 500 health, 80 ap, and the amazing slow. What do people like so much about RoA?

On some champs who have spamable abilities that chew though mana, it might be slightly better to get a RoA, though you won't get many stacks from its passive. Its usually because people like the health from an early catalyst.

the passive from catalyst is good if you're going bot, so at that point you're looking at 1710g (iirc) for 80 ability power and 180 health. you know, if you're not building into odyn's veil.
i very rarely pick up catalyst if i go top--not nearly as much appeal in that case.

Some champions need the sustain it gives, to well sustain. Other than that there's no real need for it considering the passive takes a while to ramp up and the games about to end when you do get its max potential.

RoA is usually for bot like some have mention.

The Catalyst is amazing for early game sustain, since you gain levels in rapid succession you regen frequently. Plus, The RoA is a BIG stat stick, and only gets better over time, making it very gold efficient.
And hey, nothing says you can't build it AND Rylai's. I know I do sometimes.

Cus it works great on bot lane? And yes some folks do need the mana on bot lane.