In my household it's MS Office. My partner very much enjoys the UX of KDE, but can't switch because she's highly reliant on features in MS Office that I cannot find transliterated well enough to work on Linux. Otherwise, there is no place for Windows in my house.
For me? Gaming and lack of alternatives. I hate MacOS' hostility toward anything not Jobsian, and Linux doesn't have parity with Windows particularly around gaming.