Here is a good reason: you're debugging some problem that occurs in the middle of garbage collection. You have the world stopped and would like to look at some objects. Oops, pointers are scrambled by this algorithm and won't be put back in place until it runs to completion. (Which it might not even be able to do due to whatever bug you're trying to investigate, even if you have a way to initiate that.)

With regular recursive traversal, you can just bail garbage collection at any time: loop through all the heaps, and reset the gc bits of all objects to the "unmarked" state. Then examine whatever you want.

(If the bits aren't in the type tag, you don't even have to do that; though it's a good idea for them to be in a type tag because then an object which has been mis-handled by memory management (e.g. marked object remains in the live object graph after gc) then it has the wrong type and blows up when used.)