[Not an astronomer]
It's used for stylistic effect. The fact that the earth rotates at 1,100 mph is interesting, but not as absorbing or exciting to contemplate as the fact that it is doing so at this very moment! That means that you and I are travelling at 1,100 mph. Putting it that way adds immediacy to the fact.
Also, over time, the speed may change. But don't quote me on that.
Student or Learner