Here's why:

* Homologous structures are similar in structure due to a shared common ancestor, but may have different functions. Example: the wing of a bat and the arm of a human.

* Analogous structures have similar functions but different underlying structures because they evolved independently. Example: the wings of a bird and the wings of an insect.

* Vestigial structures are remnants of structures that were functional in ancestral organisms but have lost their original function over time. They often are reduced in size or complexity. Example: the appendix in humans or the pelvic bones in whales.

* Neither would apply if the structure doesn't fit into any of the above categories.

Remnants of structures with past functions clearly fit the definition of vestigial structures.