Hardwood: how much is left
A floor sanded to the limit of its wear layer cannot be sanded again. That is measured before refinishing is recommended, not assumed from the look of it.
Matching is the usual constraint
Batch, age and wear all differ. An invisible repair into an existing floor is rarely achievable, and we would rather show you than promise you.
When repair is clearly right
A small damaged area, a sound subfloor, and material still available or salvageable from a closet.