It's used to keep time in a decentralized system, which is actually very difficult to do it turns out.

I was there with you thinking that made up mathematical problems were wasteful. But they're needed in a radically decentralized system to independently agree on the order of things.

Here's a quote from Satoshi Nakamoto's white paper:

"The network timestamps transactions by hashing them into an ongoing chain which serves as proof of the sequence of events witnessed." - SN

It's about validation not computation. Without it you the system wouldn't be able to tell before from after.

-Isaiah