> The other side of the coin is for apps that are already listed in the Play Store, does side loading have any legitimate use cases other than piracy and IP theft?

Yes - bypassing incorrect assumptions.

For example, a US company I worked for at a non-US office (imagine it was e.g. Google Japan) gave most employees a corporate credit card. Citibank made it impossible to download their app through the Play Store from the country in question, even though it was required to use it. Only option was to sideload it.

This kind of thing has happened on other occasions as well.

I am currently locked out of online banking becuase my Bank uses a security lib of questionable value. One of the "security" mechanisms is a block list of other apps installed on your phone.

In the past it was magisk in there, since I can't root my current phone I don't have magisk either. My only way to use online banking is copying the app into my work profile profile, or using an older version which I have to sideload from apkmirror.