iso answer??

you receive guild points and you can unlock new characters if you did mq with the previous class:

see here:

Fail -> No Character Unlocked -> 1 Guild Point
Feeder Class -> Samurai - 2 Guild Points for a Pass
Samurai -> Paladin - 5 Guild Points for a Pass
Paladin -> Monk - 6 Guild Points for a Pass
Monk -> Ninja - 10 Guild Points for a Pass
Ninja -> Warlock - 12 Guild Points for a Pass
Warlock -> Headhunter - 25 Guild Points for a Pass
Headhunter -> Alchemist - 35 Guild Points for a Pass
Alchemist -> Reset Season - 100 Guild Points

so what can you do with guild points?

collect a lot by climbing to level 71 fast and often
they are kind of prestige, representing the activity of a guild and of course also of a player.

since guilds are ranked by their number of gps in the official guild list:

http://forums.d2jsp.org/index.php?act=guild

some guilds also pay fg for each guild point to climb this list faster.