Cleaning out the grass clippings from under the mower deck and sharpening the mower blade will allow your machine to work more efficiently. It will mow the lawn better, faster and use less energy. That makes for a better appearance for your lawn, and saves you time and money. Repair is fixing something instead of replacing it. Sometimes only a small part is at fault, so a repair saves money and keeps the larger unit out of a landfill site.
Rebuilt parts are part of repair as well as reuse.
Retain is about keeping useful things at hand so they are available when needed to repair or build something else. Nails can be straightened and reused. Screws, nuts, bolts, washers and other small hardware can usually be reused. If you do not have a need for something, perhaps your neighbour, friend or relative does. Perhaps someone could use that sink or cupboards you are discarding in the basement they are finishing or in a cottage or workshop.