What problem does this address?

Right now, if your theme is not a block theme, you don't have the ability to create general template parts from the Site Editor. The only template parts you have access to are the ones added in your theme. It would be very helpful from a content administration perspective to be able to create custom template parts that could be used in other places.

What is your proposed solution?

I could see this being solved by the upcoming Library esque feature combined with more powerful patterns that you can create/edit without diving into the code, as well. But I do think the ability to create general template parts in the site editor should be an opt-in setting for themes. It would solve quite a few workflow issues.