A drink coaster goes under a glass or cup, and is intended to catch any condensation or spillage from the glass, to protect the tabletop underneath.
But most coasters are flat.* Any liquid that gathers on them can roll off the edges onto the table. Some coasters are made of a water-absorbing material, like cardboard or cork, but some are made of materials that repel water, like metal or ceramic or plastic.
I ask this because I recently discovered a small coaster-like tray with an upraised lip around the edge. Strictly speaking, it's not a coaster, but it's exactly the right size to be used as a coaster - and, with the upraised lip around the edge, it actually prevents liquid from escaping onto the table.
So why are coasters flat?
(I bought some of the lipped not-coasters to use as coasters. This design makes sense to me. And they happen to look nice as well.)
* It was only while researching coasters online prior to making this post that I discovered that some coasters have lips. Every coaster I've seen in real life is flat.