For why buy YOU tokens? They can be used as an internal currency within the project to purchase products and services from companies represented in the marketplace, and to pay for contextual advertising or premium accounts in SELFLLERY!
SELFLLERY is platform that connect advertiser/seller and users/buyer, they don't need that content form themselves. And if you mean why they didn't just use currency like their or bitcoin instead of creating a token, they can do that if they have the money to build that the platform, but since they need to fund their project, they started ICOs that requires to issue a token.
Anyway I read their whitepaper that YOU tokens should not be seen as an investment, but they will have some value on the cryptocurrencies market.