You don't want them getting nightmares, do you? Good sleep is important when you're infiltrating the Federation.Which is also dumb. Odo's pot must have come from the Bajoran scientist who found him, at the very earliest, if not something he picked much later. Any container will do. If I were an infiltrating Founder, I would bring a container that doesn't look like a container. But no, there is a pot very much like the one Odo was not born with.
The aesthetic of the pot made me assume it was a Bajoran thing too. Still, I don't remember what we know about Odo's Jor-El special baby pod, but I guess it's conceivable that the pot was in there and Odo had it his entire life. It is kinda dumb.
It would have been better if Seven just found a standard Starfleet beaker in that light fixture, and it would have served the same confirmatory purpose for both her and the audience, because Seven does identify the residue after she finds it. But the writers clearly wanted the audience to go "aha!", and I can't really fault them for wanting something visual to put on the screen rather than relaying plot information through a tricorder reading. Much as how all interaction between starships in external shots scales down the distances by a factor of 100 so we can actually see both ships at the same time.