At the time, mi5 was a startup with a neat idea. Today, it has been acquired by Symantec, but it still is "Your first line of web defense." The Webgate no longer is mi5's only product, and the company's product offerings are more sophisticated and cover a broader landscape. But the basic idea of the Webgate is still solid: Keep the bad stuff out if you can and, if it does get in, don't let it steal from you.
Most of mi5's newer products are designed to be part of the Webgate platform, which is just fine. As I have said many times: simple usually is better. The refinements added to Webgate over the past few years are well thought-out, which is yet another thing we all like to see in a product. Mostly, these refinements are added as additional, optional modules. This makes an ecosystem that you can mold to your infrastructure and regulatory reporting needs.
The basic idea behind Webgate is that there are a lot of infection vectors for bots and other malware, but the most dangerous - because we are stuck with it and cannot shut it down even if we wanted to - is the web.
Users are going to use the web and they are going to go to places that they shouldn't. That means that they have a good chance of being infected with some form of bot or spyware that wants to harvest sensitive information and send it to its controllers somewhere over the internet. We really don't want that to happen.
So, mi5's products are focused on protection in both directions: in and out. This is real defense-in-depth. Data leakage can come in many forms and Webgate does not pretend to have them all covered. For example, one could steal data with a thumb drive. But that's for another type of protection. For web-based leakage threats, Webgate is just the ticket.