A: There's a few reasons, such as when you buy a new SD card to make sure it isn't fake. Or if you were having issues with installing games, and they kept corrupting themselves, it's a way to make sure your card isn't going bad and helps to narrow down issues.
Note that "full formats" for FAT32 can help, however they're not an automatic "fix it all" solution. A quick format doesn't check for bad sectors, where a full format (slow format often called) does check sectors, however you don't know how many bad sectors there might be, or what condition your drive is in making it pretty pointless. Using a tool like Flash Drive/Card Tester will show you how many bad sectors you have, giving a better idea if the card might still be salvageable, or if you should just replace it outright.
Note: If you were using your SD card before, this will erase everything on your card. Back up contents you want from it before doing this, I am not liable if you lose your saves because you didn't back them up. See these guides for how to back up different stuff on your switch.
Step 1. Download Flash Drive/Card Tester v1.14 from here.
Step 2. Install the program and launch Flash Drive Tester v1.14 after it's installed.
Step 3. In Select flash drive/card, specify your SD card.
Step 4. For Select test type, specify Write, read and compare. Hit Start test after. This will take some time depending on the size of your SD card (up to 2 hours per 100GB~).
THIS WILL ERASE ALL CONTENT ON YOUR SD CARD. IF YOU HAVE NOT BACKED UP YOUR SAVES, DO THAT AT MINIMUM BEFORE CONTINUING. USE CHECKPOINT, OR JKSV TO BACK UP YOUR SAVES.
See this section of my guides for how you can back up different parts of your switch if you need. You do not need to back up your game saves if you're using sysMMC, this only applies if you're using emuMMC as the utility erases your SD card (which would contain your emuMMC if you have one). Other files/folders such as Atmosphere, Switch, etc. you can just copy off the SD card somewhere safe on your computer.
Step 5. Wait for Flash Drive/Card Tester to finish, if your card is good it should have no errors at all, looking something like my image below. If you have errors, you can try a slow format FAT32 using FAT32 GUI format tool, see this page for how you can format your card to FAT32. Make sure you uncheck Quick Format if you do this. Afterwards, test your SD card again, however it's likely that it's already failing.