Some professors prefer to submit letters via mail, because whatever aadsas (or any other such service) sends them ends up lost either in junk email folder or among 100000 other emails they get.
Others prefer paper because they think it is more professional.
I gave my professors a choice between the two ways, and 2 out of 4 of them chose to do it via paper because of the above reasons. There is as much trouble associated with mailing as with e-mailing, imho =)