The use case for me is mostly hotel stays. When travelling for fun I don't particularly want the weight (and risk) of packing my laptop, but I would like to be able to play movies and music on the hotel TV. With a cheap light keyboard I could also do email and similar light admin things with the benefit of the bigger screen.
Until recently I was also travelling frequently for work with a heavy MacBook running a variety of Enterprise malware that would prevent me from doing any personal stuff with it - being able to use my own phone for light leisure activities that I normally do on the personal laptop would have been very useful for the boring hotel evenings. Adding another kg+ of laptop to the already heavy backpack was too annoying to put up with - my cheapo bluetooth keyboard and mouse weigh in at a much more acceptable 280 grams.
In short, there's a use case - but most phones are too locked down to take full advantage of the possibility.