In many ( most) cases it's impractical for people to buy tickets at the venue. Even leaving out situations where tickets sell out quickly the costs associated with going to a downtown box office tend to exceed the fees annoying as they are--and even in person there tend to be various facility fees and the like.
It can't be fees for buying in person, since I'm being told that buying in person with no fees is how they can get away with advertising a the base price. As for the inconvenience of buying in person, that really doesn't seem to add up, but I suppose people really could prefer to spend an extra $60 over going to the venue in person to buy.
