Promotion.
If people favorite your item, that helps your relevancy. And when someone favorites your item, it goes onto their activity feed so the people that "follow" them can see your item[s], so it's potentially a chain for promotion.
These are other examples:
-If team members are creating a "team" treasury, they can browse through the items people are promoting for their treasury.
-Depending on the team...for example a "Christmas" team - there are different categories 'gifts under $20', 'for him', 'for her...some people [team members and not] browse these for gift ideas.
So it can be free promotion and beneficial.
Hope this helps.
Cheers!