What effect does the author achieve by including the detail about Minijogger and Rex being friends?

Question

 The mouse jogged quickly on the keys.
This was his nightly exercise"Hey, Minijogger, that sounds nice,"
said Rex the cat about the tune.
"Thank you, my friend.

Answer 1

If that detail were not included, what would you assume about the relationship between a cat and a mouse? This could help inform your answer. If you would like more assistance don’t hesitate to reach out.