Every reseller package is given a portion of disk space to be distributed amongst hosting accounts. Remember, most basic Web sites don't require a great deal of space, but if you have sites with a lot of media files (pictures, video, Flash, etc.), moderate-to-high email volume and databases, your disk space can fill up quickly.
If you can anticipate your needs (or your customers') needs well, then you can purchase a plan with lower amounts of disk space confidently. Remember, we can always upgrade you later if you're building up enough volume within your account (a good problem to have).