I've been using Forté [sic] Agent since it was in beta back in 1994 or so. Back then, Usenet was more important than email, and Agent was a news reader that had an email client built in. As Usenet faded in importance, and email skyrocketed, Agent got smarter and smarter at email, and now includes a lot of features that no other client supports. But at the same time, it's still tied to some underlying design decisions that made sense back in the days when POP3 was it for email, which makes implementing some features that are now standard in email, like IMAP, harder than they could be. Unfortunately, with email clients typically selling for $0 and Usenet clients all but extinct, Agent isn't really a selling product anymore, so Forté isn't doing more development. They kept it going long past the point it didn't make a lot of sense, but they seem to have hung it up.
IMAP's main claim to fame is that your email and folders live on the server, in addition to (or even instead of) on your client. That way, you can see it with multiple devices and they stay readily in sync. This isn't really done perfectly -- it still uses a nested-folders approach instead of a tags-like approach, for instance -- but it's good enough, and it's now the standard. Just about every email client, including those on smartphones and other portable devices, supports it. Thus, you can do some email management on your phone and have it be reflected when you get home to your computer.
With POP3 the best you can do is have two computers get the email (by using the "read" flag as a cheap way for them to keep in synch), but if you have folders, they are on the client, so you have to have the two computers handle the folders identically. And you can't get to a third client this way. And not all clients even support this kluge.
For a lot of people IMAP was a must-have ten years ago, because they had multiple computers (home and work, or more), and especially because they had portable devices with network access. But I haven't really needed it. Phones with data plans were rare in Vermont, where most of us still have to choose a provider based on which one has signal at our house. And I have been using the same computer at home and work for many years (and if I hadn't, I'd've been using a thumbdrive or SDcard for my email data file anyway -- as I am already doing with my Eee).
But I have a smartphone with a data plan now, and sometimes I get annoyed at forgetting to shut Agent down when I'm away from my computer, and finding my phone can't show my email. And of course I can't delete, file, or do much of anything with email on my phone and have the changes reflected when I get home, like I can when my phone hits my work email. I don't even get copies of my sent messages unless I BCC myself. I've also dabbled in accessing my email from my tablet from time to time, so that's even more clients.
All this is pushing me towards making the move away from Agent. Word is Thunderbird is the client of choice for Agent expatriates. So I'm going to look into how well I can make it work for me.