What is the practical benefit to putting t.Parallel() at the top of my tests?

The go testing package defines a Parallel() function:

Parallel signals that this test is to be run in parallel with (and only with) other parallel tests.

However when I searched the tests written for the standard library, I found only a few uses of this function.

My tests are pretty fast, and generally don't rely on mutating shared state, so I've been adding this, figuring it would lead to a speedup. But the fact it's not used in the standard library gives me pause. What is the practical benefit to adding t.Parallel() to your tests?