If a clogged drain line is not addressed, it can absolutely result in a water damage event. If you know you have a clogged drain but keep using the fixture attached to that drain line, whether it be a sink, toilet, tub, or shower, the result will be that the drain water will seek its own level. Often, this leak will be into a basement, crawlspace, around the base of a toilet, or in the shower basin. Once water seeps past a toilet seal at the floor, that seal will be prone to leak from that point on and should be changed. It is possible for a toilet seal to leak and never show on the floor, showing up below in the basement or crawlspace. When the stoppage is removed from a drain line, it is a good idea to be sure there are no resulting problems with the piping that can cause damage later.