Well think of it this way.
Which is better for you when starting up a new company.
A $100+/month dedicated server, or a reseller package for around $10+/month?
Plus, you get the server reliability of the root host, you save money (money that you wouldn't have saved if spent on your own server).
To be honest, a reseller is meant for new companies.
Big companies, or successful companies, would have their own servers. So apart from the 3 above, there aren't really any others I can think of. Those 3 are what reseller packages are meant for.
Perhaps, ease of use could be another one. You don't have to worry about root access and installing/managing the server. You leave that to the root host.