It's a store you can get in your home city that sells chests. Lots and lots of chests. They're broken down by level, so that you can buy things appropriate to your level. It's basically gambling, you hope that the chests you buy have items in them that are useful to you or worth more than the box.
As for where to go...many, many user cities. Pick the person whose spam you find least annoying, or if you have a friend with one, use his.
As for "why do they spam them"...they get 5% of the cost of each chest back.