Why 240p, 360p, 480p, 720p, 1080p ?

The web video script I'm looking at getting apparently saves an uploaded video in 240p, 360p 480p, 720p and 1080p, via ffmpeg.
That seems like overkill and it seems that it would take up a lot of disk storage to store 4 copies of each video.
The uploaded videos don't have to look state of the art when viewed. If I wanted to reduce the potential storage, if I were to modify the ffmpeg to not upload so many version the each video, which resolution(s) would be best to keep? What is the downside…

Why 240p, 360p, 480p, 720p, 1080p ?