On early phones, taking the SD card out of your phone and putting it in your computer was a common use case. It was usually a much faster way to get files onto it than tethering the phone by USB was. And those old phones had card slots that were right on the side of the device so the card was easy to remove and replace.
I remember doing that for my first smartphone (Sprint Evo 4G); the SD card was a pain to get to (you had to take off the back cover and remove the battery) but the phone's SD data transfer was very slow so you still came out ahead if you wanted to fill the entire card. And on that old version of Android, tethering the phone and mounting the SD card on your computer unmounted the card from the phone, which made my phone nearly useless because I had transferred as many apps as possible to SD, something you could still do back in the days of Gingerbread. So turning off the phone to do the SD transfer wasn't a major imposition.
Modern Android devices don't actually mount as a file system on your computer; they instead use MTP (media transfer protocol) to make files visible over USB. (The computer's OS may display the MTP device in a way that LOOKS like a mounted file system, but that doesn't change the fact that a different mechanism is used under the hood.) That eliminates the problem of having to unmount the SD card from the phone. It's not in theory any faster, but new devices usually have much faster USB implementations so there is no real speed advantage to taking the card out. And the cards are usually still buried in some location where they are hard to take out, unlike tablets and media players that typically have them accessible from the outside.