Why are Shocklands better than Guildgates?
Shocklands (which allow you to "Shock" yourself in order to get an untapped land) have many advantages:


Good color depth without loss of tempo. Oftentimes, getting your land into play untapped is worth two life; shocklands give you the option to set that up.
Excellent color-fixing with fetch effects.


The synergy between shocklands and fetchlands is what makes Modern mana bases so strong. I can crack a Misty Rainforest and get any of these color combinations: W/G, W/U, U/B, U/R, U/G, B/G, R/G. See this answer for more information about the shocks + fetches style of mana base.
We can have four- and five-color decks in Innistrad/RTR Standard thanks to Farseek's ability to fetch any of the ten shocklands.

Shocklands interact positively with other cards that care about basic land types as well. In particular:


Shocks power up M13 and Innistrad "buddy" lands (like Sunpetal Grove), which require you to have lands of specific basic land types in play. In a three-color deck, a single shock in play is sufficient to make every single one of your "buddy" lands enter play untapped.
Shocklands have powerful synergy with "Kird Ape" creatures like Wild Nacatl (which was once banned from Modern for being too consistent of a 3/3 on turn 2) and Flinthoof Boar (which saw tournament play in Standard Gruul decks).