Because you want to be able to dynamically load semi-trusted code on a microcontroller without MMU and run it without having to worry it could crash the whole thing?
Do you have an actual example of such a thing? I'm struggling to come up with a case of multiple independent mini-programs in such a microcontroller that needs, or even benefits, from that level of fault isolation.
Rapid prototyping, replacement for a scripting facility, diagnostics filtering & output, a platform for "apps" for a small IoT type devices and just the ability to run the same plugin anywhere you please.
It can be an enabler for new innovations. Even if it runs 2-10x slower than native code.
And what if the answer was just: just because I can? That's how a lot in our industry got started.