When your business is heavily dependent on its Internet connectivity, downtime can equal lost productivity (and lost money), and performance is also a big issue. The ISP redundancy feature built into TMG is a great way to address both of those issues by giving you a way to connect to two separate ISPs through the TMG server. The connections can be load balanced to provide better bandwidth, and/or you have failover capability in case one of the connections goes down.
There are limitations to this functionality, and sometimes you run into problems when you use a less expensive service as the secondary connection. But there’s a way to get more out of that backup service, and this article over on the FastVue blog explains how to do it:
http://www.fastvue.co/blog/extending-forefront-tmgs-isp-redundancy-features