I guess they figure that having resellers will spread out their market share better and it makes it convenient for more people that way. and, why compete with the resellers...
Actually there are quite a lot of processors that do it that way.
Then you can log into the authorize.net web site with that password and set up your stuff.. such as numerous configurations, custom logo for your check out pages, link back to your site after check out, and tons of other things depending on any extra packages you get which are add-ons billed monthly.
Watch all the videos and it will make more sense.. They do have a lot of short and to the point videos.. Some of them don't get into full details, but it gives you a good idea of what's up.
I guess I should add that you would need a Merchant account with your bank as a place where the authorize.net gateway will push your funds too. A merchant account is just a checking account that will take on-line transactions from any gateway processor. If you take in-store transactions, you most probably have a merchant account now.
Paypal doesn't require a merchant account, but you would verify your account to hook it to paypal if you go with the automatic deposit deal.