When you first start a game, you start by working the very best tiles, often only resource tiles.
As the game progresses, you use more and more tiles and then you use the best ones first and save the worst ones for very last.
Circumstances may vary, and not all cities are equal, but the rough prio order is:
resource tiles -> floodplains -> riverside grassland -> normal grassland -> riverside plains -> plains. And after that ofcourse you have tundra, riverside ice etc...
So generally, plains tiles are ignored for much of the game.
IOften pretty much up untill 1000Ad - 1500 Ad mark or when you start to get guilds+chemistry+caste system workshops on plains tiles start to look OK in food dense cities.
With state property workshops on plains tiles are good in most cities.
It's pretty rare, but sometimes a stray cottage on a plains tile makes sense, but the key element is that you must really be completely deprived of other more useful things for your workers to do before resoring to such actions.