"Why is it important to 'format' the card in the camera rather than in a computer?"
The main reason for this is every camera brand formats differently and every camera is different. Also every computer formats differently to. So even though your card might be readable, it could well be corrupt and eventually cause you issues such as lost or faulty photos. Case in point I did a wedding last week and the second shooter used one of my cards, however lightroom could not read the RAW files, even though their camera was a canon like mine because the card was not formatted in their camera, it caused issues. I was able to convert the images to jpeg and rescue them for editing, but it could have been a disaster, something you don't want with wedding photos.
If you format in the camera you are going to use, it is a clean format and formatted to use in that particular camera. Better be safe than sorry.
BUT... remember to back up your photos to at least 2 places before you format the card. Once formatted its nearly impossible to recover the images, although in some instances it can be done.