Because a desktop isn’t portable?
When I go into either one of my corporate offices (rarely) or a client’s office, it’s nice to be able to just plug my laptop in the night before and not have to worry about a finding any place to plug it in during the day, going back and forth between conference rooms, etc.
I also go home to see my parents for a couple of weeks sometimes and work from there.
And I realize this is a very esoteric case. But I also do the whole “digital nomad” thing half the year.