I’ve been working with a fly lab the last few years. They really do model diseases and gene functions. There are tools to look up genes across species and determine how close they are (othologs). Plus cheaper to study.

For example circadian rythmns were discovered in flies first:

https://www.nobelprize.org/prizes/medicine/2017/press-releas...

Disease research also is done:

https://dmm.biologists.org/content/9/3/235