There are many reasons why you might want to work tempo and/or time signature changes into a track. The most obvious one would be to dramatically change the feel of a song; switching from 4/4 to any odd time signature is about as dramatic a temporal change as you could ever hope to pull off mid-track. And the signature to which you change doesn’t have to be an odd one, either. Throwing a ‘fill’ bar of 2/4 into a 4/4 project can be a more subtle way of throwing the listener a curveball.
Tempo changes are considerably more varied in their potential usage scenarios. Of course, like time signature changes, they can be just the thing to push a track in a totally new direction, whether applied suddenly (blasting into a half-/double-time section, perhaps) or gradually changed over a number of bars.
Some DAWs feature the ability to slave the project tempo to that of a chosen audio clip, enabling, for example, a live drum loop that varies in tempo to be set as the master for every other track in the project to follow (assuming the DAW in question also facilitates automatic timestretching of the audio on those tracks). Gently raising and lowering the tempo as a track progresses (known as ‘rubato’) in order to add pace to choruses and ease off in the verses is an age-old technique dating back centuries – originally the job of the orchestral conductor, these energy-giving fluctuations can now be drawn right onto the tempo track in your DAW.