The 3G model was for playing vita games in online multiplayer on the go, and little more. This is clearly obvious. Just ask KBG, he actually tried to use that thing as a phone.
A small correction: the 3G model wasn't intended for online multiplayer games... you needed Wi-Fi for that in most cases. Only a few games offered online multiplayer via 3G.
3G was mainly used for non-gaming stuff (internet browser, E-mail, GPS navigation,...) and for cloud syncing savegames.