Job-site applied oil or water based polyurethanes require sanding off old finish and reapplying a new finish on unfinished an unfinished wood surface. Some of the wood surface will be sanded off as well. Factory applied finishes are compatible with the common water-based acrylic polyurethanes now widely used in favor of oil- based finishes. So a re-coating no longer requires the factory-applied finish to be removed, but merely abraded sufficiently for the new finish to adhere. This is much less of a mess than a full sanding, and does not remove any wood from the surface of the flooring, unless you’re dealing with dents.
There are two things that accumulate on a wood floor after years of use. One is dents. Dents are simply a function of the intrinsic hardness of the material and the weight of whatever drops on the floor. Additionally, spiked heels, gravel, or any sharp object will likely leave dents in a wood or bamboo floor. The other accumulation of history has to do with the surface finish. These are primarily scratches and gloss reduction in a high traffic corridor. Both are exclusions to the finish warranties. Finish issues are remedied by refinishing the finish. If there are isolated dents, it is better to replace those pieces rather than grind the whole floor down to the deepest dimple.
Having said all that, wood floor finish manufacturers are developing the chemistry to re-coat wood floors without any sanding or abrading. Cosmetic re-coating in the near future might well be applied with a mop and a spritz bottle available at the local supermarket.