There are 2 reasons. First, the chemicals in the polyurethane and the chemicals in the rubber can interact and cause discoloring (usually yellowing or darkening) on the floor.
Secondly, and this is true with all rugs, the light will naturally change the color of the wood. If there is a rug atop of it, it will not change like the rest of the floor. If you move your furniture often, keep this in mind.
A hardwood floor requires regular maintenance, the extent of which is determined by the amount and type of abuse it endures. Protect your hardwood floors from indentations and gouging by adding felt pads to the bottom of your furnishings. Add pieces to the bottom of each foot on a table or chair, or you can add strips to the edges of cabinets. You can also place furnishings on throw rugs. Remember to add rubber base padding under each rug to avoid slipping. Protecting your floors will reduce the extent of upkeep needed, as well as the cost that such maintenance will incur.
Rubber casters are better for a hardwood floor as they will not scratch the wood surface and not leave black marks on your floor.
Rubber dumbbells are much better in places where you have hardwood floors or other uncarpeted surfaces that you are concerned about damaging. They generally make less noise and will not scratch surfaces when they are set down. Additionally, if the handles are solid rubber, this is more comfortable to many people than metal handgrips.
A church has a rubber tile floor that needs serious cleaning. Can I steam mop it?
There are a variety of Casters wheels that work on ottomans. One should instead look at the material that the ottoman is being rolled over. Metal or sharper wheels may scratch hardwood floors, while rubber may scuff tile.
Rubberwood is a hardwood. It is similar in characteristics to Maple and Ash. It has little tendency to warp or crack. It is cosidered eco-friendly, as it is only harvested after the Para rubber tree has completed it's lifetime growing cycle.
Clear rubber stamps are made from acrylic and you are able to stamp accurately and with perfect precision, something wood block and foam-backed rubber stamps can't offer
Rubber Boots
for this answer you have to ask your local city home inspector
Aside from power washing, there is acetone (nail polish remover). Acetone will help to break down the 'rubber'
To remove rubber padding that has adhered to a hardwood floor, use a very thin wallpaper scraper, proceed slowly and finish by lightly sanding the area. The finish of the floor has most likely been dissolved by contact with the runner padding, so the finish will need to be renewed.
So long as the floors are level and there is no drainage issues then putting rubber mats over dirt floors is perfectly fine.