Why spend Bitcoin Cash (BCH)?

Two reasons

Firstly, you gain experience, making it easier to go with Bitcoin Cash (BCH) as payments when other possibilities become closed.

Secondly, by spending Bitcoin Cash (BCH), you signal to the vendors that the coin is useful and having Bitcoin Cash (BCH) prominently displayed on the web page can draw a few new customers. And indirectly you signal to other users that BCH is a viable payment option.

Both of these spread the word, it encourages others, and thereby also increases the value of your remaining coins.