Some stores will offer a 10% discount if you're a guild member, so maybe that's why they ask you.
I've been members of 2 different guilds for many years, and think it is mostly a rewarding experience. Yes there are people there who aren't my first choice of friends, but then there are wonderful people there who will do anything for those less fortunate and offer help any time it's needed. I also belong to a group of 8 ... all originally met through my guild.