When an Excel document grows enough legs, just sometimes someone will decide that it's useful enough that they should build a proper service to replicate its functionality. That's the point at which so-called "real" programmers want a debugger in Excel: because someone with talent but lacking experience has built a monstrosity that now needs to be properly replicated and made maintainable.

Especially when the original developer's manager starts asking pointed questions about how their one "non-programmer" could build something in Excel that then takes a whole team ten times as long in a proper programming language.

Having a debugger available to help reverse-engineer the spec would be really useful.