Well, you could use that argument for learning anything, really . . . english, math, knitting, cooking, etc. I've personally found that in person is always better than from the internet.
Plus, it can be hard to motivate yourself to set aside time to learn something new. There's always cooking, cleaning, laundry, a new movie out, a forum to lurk on, what have you. Lessons make you set aside time every week to do something, and it's proven that you practice more if you have a teacher.
Then there's repertoire. I have access to a lot of music that it would be hard to find on the internet, particularly if you're an English-speaking person looking for kids songs in dialekt. I also have access to a bunch of cool folk songs in English that people might not know to look for.
So yeah, lessons are a great thing, for motivation, quality, and the social aspect (particularly in a group setting). That is why people pay professionals to give them. Same as a math tutor, except more fun.