Why does ActivePython exist?

It's a packaging, or "distribution", of Python, with some extras -- not (anywhere) quite as "Sumo" as Enthought's huge distribution of "Python plus everything", but still in a similar vein (and it first appeared much earlier).

I don't think you're missing anything in particular, except perhaps the fact that David Ascher (Python enthusiast and my coauthor in the Python Cookbook) used to be CTO at ActiveState (and so no doubt internally pushed Python to go with other dynamic languages ActiveState focuses on), but he's gone now (he's CEO at the Mozilla-owned firm that deals with email and similar forms of communication -- Thunderbird and the like, in terms of programs).

No doubt some firms prefer to purchase a distribution with commercially available support contracts, like ActivePython, just because that's the way some purchasing departments in several enterprises (and/or their IT departments) are used to work. Unless you care about such issues, I don't think you're missing anything by giving ActiveState's Python distribution a pass;-).

_{(I feel similarly about costly Enterprise distributions of Linux, vs. Debian or Ubuntu or the like -- but then I'm not in purchasing, nor in an IT department, nor do I work for a very traditional enterprise anyway;-) )}

The main feature is that you can buy a paid support contract for it.

Why does Red Hat Enterprise Linux exist when you can compile everything yourself? 8-)

For many businesses, the combination of proven Open Source software and a support contract from people who build, package and test that software, is an excellent proposition.

ActiveState has a long tradition contributing Windows support to Python, Tcl, and Perl: by hiring key developers (like Mark Hammond, for some time), by fixing bugs specific to Windows, and having employees contribute fixes back, and by being sponsors of the Python Software Foundation.

While it is true that the distribution they produce is fairly similar to mine, it's as RichieHindle says: you can get paid support from ActiveState (but not from me).