Tom Forsyth listed one of the reasons--when a storage is explicitly expressed. The other two that I can think of are for external linkage and non-constant expressions on the rhs. In the end, you really don't know without some introspection. It's up to the compiler. External linkage is one that will force storage, but in general const will require internal linkage and fold expressions accordingly (at least for primitives).

Edit: I would never expect anyone to actually remember or need to know that for an interview. It would be definitely something you would re-discover/discover because you had to run into it.