What is the purpose of the toggle switches under “cellular“ in the settings menu?

If you are referring to the switches by the app names (e.g., Safari, Maps, Notes, etc) in the Cellular Data section, if those switches are on then that app will use your cellular data when wi-fi is not available. If they are off, then that app will not connect to the internet if wi-fi is not available.

The basic purpose is to let you control which apps are allowed to use your cellular data, so that you can, for example, avoid exceeding your monthly allowance, if there is one.