You are supposed to put them on shelves (bookcases to be purchased from your local store) and dust them off at least once a week!!-) Seriously, the XPs, lingots, levels and fluency scores look like incentives/carrots for encouraging people to come back for more... Some of them do attempt to provide some kind of measure for how well one is doing in a specific language.

I have 100 lingots, I have no idea how I got them, but they are pretty useless unless you want extra courses on a language

Yes, I'm also impatient to spend my lingots; in the Hebrew course the selection of items is even less (two outfits for Duo, streak freeze and double-or-nothing). They've just added new ways to earn more lingots, so I'm half hoping that they'll soon expand the shop a little as well.

I think they're basically for giving to posts you really like

Check this discussion out: https://www.duolingo.com/comment/24704962

hi can i join

i have done that