Steam Vents

For starters you don't have to pay the 2 life, that is only if you want it to come into play untapped. Next this counts as an Island AND a Mountain. This allows players that have cards that say "Search for a Mountain" or "Search for an Island" they can grab this land instead of a basic land.
Radiant Fountain does give you 2 life but it will only give you colorless mana, it doesn't give you anything else.

Shock lands looked bad to me too when I was new, but they are amazing. On their own, they're kind of pointless, but with fetch lands they are incredible.

The main reason they're so expensive is because of Fetchlands. If Fetchlands didn't exist, Shocks would more than likely be bulk price. And like RolledAOne said, it counts as 2 Lands (ex. Mountain Island) so you can use something like Wooded Foothills, which only can fetch Mountains or Forests, to fetch up a Steam Vents, which counts as a Mountain so it works.