It's just the system used by my school. The school's office scans all my rec letters into digital form and simply upload it to the med schools that I request.
The pros SHOULD be that this process is faster and letters won't get lost in the mail... but there are cons, like, schools (as here) that are dilatory themselves in downloading these forms. I suppose when it's on paper, they are more compelled to file them. But also, not all med schools have converted to virtual evals yet (although most have), and so i personally think it makes the premed office confused. For example, they forgot to mail the hard copies to many of my california schools since those aren't on the virtual system.
It's not a system you choose, I think. It just depends on whether your school does it.