Most sites don't need unlimited disk space, and you can offer other services to your clients - like webdesign or advertising.
The technical reason is because of our inode limit. An inode is a count of every file and folder. Shared plans have one primary user that adds content. Reseller plans, however, can have many users, all adding content and creating more files and folders. The higher the inode count, the higher the file system overhead and more resources are used.
