Not the best part of my career, but certainly one of the most helpful. I currently have a huge dental bill which without AJ would make things a little trickier (especially as its tax time as well!).
AJ has allowed me to have a residual income that allows me more time to focus on my long-form custom/original music commissions, such as video-games and films. When you start out, generally a lot of the longer 'artier' projects don't pay much money so you have to find a balance between earning a living and working on projects that will generate buzz for you as a composer. You do that long enough, and eventually those low/no budget projects early on in your career that you took on, perhaps the assistant director of that short film goes on to direct a commercial, perhaps the producer of that short film ends up producing a TV show, the director of that indie game ends up getting a publishing deal, etc. etc.. You just never know where working cheaply can lead to in the future, as long as you're not getting exploited.
This is not to say that I couldn't have done all these projects without AJ or other music library income (I work for one of the boutique library companies and write music that is NOT royalty-free), but the residuals that come in every month from Audiojungle, and every 3 months from BMI, have certainly made things a lot easier and as a result I can take on the really low-paying projects without worrying too much. The only thing that doesn't allow me to take them on is my schedule (I have a couple of high-paying long-form projects going on too, which of course, came from cheaper projects...) Doing any film/TV/game work - high budget or low budget - generally allows me to interact with real people face-to-face, which library/royalty-free music generally does not. Once you do that, you can generally 'reach' more people directly and increase your chances of success / getting more work in the future. I'm probably old-school in this regard, because I know for sure there are plenty of fairly successful film/TV composers who have never met their directors in person, but thinking LONG-term, I personally believe it's better to establish the human connection rather than just be in front of a screen all day. The entertainment industry is generally run by groups of friends and it is much easier to get hired by your friends. You only have to look at how many successful composer/director relationships there are. Most of these relationships are more than just business, they are personal friendships. I think this is something that a lot of younger composers underestimate.... BUT .... not every composer wants to do this, and there are ways of making lots of money in music without meeting anyone at all - just look at the Top Authors here!
I'm just detailing my approach, because personally, I do not get great personal satisfaction from selling music on Audiojungle (I do not have the same thrill or buzz that gballx has). The thrill I get is when I watch people play a game, or sit at a screening of a film with my music and see if people are actually engaged or not. I prefer to see the end result, the final production, and how it affects people, and I like to see it in person. That's not something I can get by Youtube comments, Facebook Likes, or AJ sales, because it seems very 'impersonal' to me. But again, that's just 'me'.
In summary, residual income has allowed me to work the way I want to work. AJ is not the 'best' part of my career, but it is certainly the most helpful. It's basically a virtual assistant that generates money for me and finds ways to put music in places that I previously wouldn't have access to. I don't pay much attention to it (I pay more attention to the forums than my profile) but I do appreciate it.