Look at the definition of flood and you will know how floodplains are made.

Now look at this picture and see a sensible use for floodplain land (other than farming). Many towns use their floodplain land for sports fields and other kinds of recreation. Then, when flooding occurs, it doesn't matter so much.

But other towns are less aware and allow houses to be built on floodplains. Then the houses get flooded.

But it is more serious than that. The floodplain become a natural lake during floods. a kind of temporary reservoir. If there are houses all over the floodplain, it cannot be a lake and so the water rushes on, flooding places farther down its course. It is far better to let these temporary lakes form than to try to block the water from spilling over its banks.