SASKATOON9999
Lots of reasons, spreading risk, managing cash flow, compliance with banking regulations.
Nearly every company that trades on the internet is using some form of clearing service.
Edit.
To explain what I mean, its worth having a look at e-clear web site. At least it gives a bit of an insight as to why these companies exist. All of the UK banks I have dealt with require the use of an approved company like eclear for credit/debit card payments on-line. Here is the link
E-Clear