I buy manga because sometimes the scans on the internet are just from the magazine and not from the volumes released, which means the chapters have other pages added in. Also, there are some manga that get licensed and you can't read on the internet. So if I see it on the bookstore shelves and I enjoyed it, of course I buy it. Besides, as others have said, it is nice to be able to crack open a manga volume in your hands since you won't always have your computer nearby.