Making use of the same bits of code on both the client and server (or whatever platforms you need to support). So if you have, let's say, some code that formats relative dates, Haxe allows you to write that code in one language and reuse it on both the server- and client-side, whereas normally you'd have to write a separate version of that in Python and in AS3.
If you have a client and a server, you'll most likely need to handle input or state twice. You shouldn't trust the client but you also don't your user to depend exclusively on the server (you want less latency and/or more informative errors).
