They were used to run any of the thousands of serious applications that the Amiga lacked, like the Lotus 1-2-3 spreadsheet, Autocad, TurboPascal compiler, various business software (invoicing, payments, etc.), even games that haven't been ported to the Amiga. I have been using PCx (a software emulator) for some time on my 030 A1200 to play the original MSDOS Tetris (still my favourite version of Tetris) and for BBS messaging (Amiga lacked for a long time a good freeware Bluewave reader).
Varthall