As an idea box mostly. I create boards for projects be it work or personal. For eg, recently I was working on designing the checkout flow for a digital product. For visual design, I created a board and put in ideas that were unique in some ways. After about 10 pins, pinterest gets good at automatically suggesting similar ideas. So it evolves without me searching by myself.
On the personal front, recently redesigne the kitchen of my parents house. Had a particular design style in mind, so I collected photos pertaining to the materials to be used, the fit, finish, pattern of countertop etc. After this, all i had to do was share the board via whatsapp to the contractor. It gave him a holistic idea of our vision.