https://www.wired.com/story/game-of-thrones-winterfell-battle-tactical-analysis/#comments

This move, sometimes known as a “Custer,” predictably ends in ruin for the Dothraki cavalry.

The forces of man are led by idiots. It was only a miracle they won. Oh for a Randall Tarley or Tywin Lannister or even a Robb Stark.

My theory is that the cavalry was used, along with the trebuchet volley, was used to take out the wight giants. There were at least a half dozen when the wall was breached, but only one made it to Winterfell. Using the cavalry to take out the siege engines seems like a good use.