Once upon a time, CCPs [Community Content Programmes] were an excellent way to get into writing for RPGs. They can still be, if you are fully aware of the implications.
If you write for a CCP you will not be able to contact your customers to tell them about new releases as a publisher can. You will not accumulate Publisher Promotion Points which can be used to run a Deal of the Day. You are frequently giving away your intellectual property, and giving permission for the publisher to reuse your content as they see fit with no reference to you.
The positives are that the CCP owner often gives you access to their intellectual property, art assets, and layout templates and they should be working to bring you an audience for your book.
Writing for a CCP should be the fastest way to make your first fist full of dollars in RPGs.
Do not be mistaken though, CCPs are built to help the publisher, not the content creators.
Over the past 12 months, possibly longer, the volume of sales through CCPs has been falling. They feel past their prime.
When David Silver of Ponyfinder fame applied to set up a CCP, he was declined.
Most of the newest CCPs have seen little activity. The rate of new title releases is slowing, and the volume of sales has been too. That makes sense. If there is little new content, why would the customer keep coming back to check it out?
The only CCP that may be bucking the trend is The DMs Guild, and that has a different issue. All the prime screen space on the guild pages is being dominated by bigger budget professionals and semi-professionals. The magic ingredient is advertising reach. If you can engage the best YouTubers, run a high profile Kickstarter, and have your own mailing list of thousands, then you can propel your title to the top of the hottest lists, and the added exposure will help keep you there.
As a smaller, indie writer, you do not have those assets, yet.
If more publishers, even those with a CCP, turn their backs on them. This is what it looks like for Monty Cook Games, and going down the SRD route takes away another reason for using the CCP.
Using an SRD, System Reference Document, means that you can write for the game system and publish anywhere. If you publish under your own publisher account you may well sell fewer copies initially, but you will start to build your mailing list on DTRPG. If you have even a basic website with an email sign-up form, you can build a private mailing list. This means that you can tell fans about your releases on DTRPG, Amazon or even retail at places like Exalted Funeral and Kickstarter Campaigns.
Each sale should, in theory, outperform the previous release because you will have slightly more reach. Each title is not starting from zero each time.
The advantage for the publisher of the core system is that each new release publicizes its core game. This is free advertising for them and is totally hands-off.