Customer support - help - documentation
Developers might think that customer support is not a fancy thing, but it turns out providing help to the users is sorely missing in most open source projects. Both in large projects such as Moodle or VLC and in smaller ones like mdbook.
There are always people who don't know how to do thing with the software or who encounter things that don't work as they expected. Each Open Source project has some forum where people can ask questions or report problems. Figuring out what is the solution and verifying if reported problem is indeed a problem with the product takes a lot of time. Taking that off the hands of the core developers will help them a lot, and it is also an excellent way to make yourself familiar with the application and the code base.
In many cases the reported issue comes in because the user did not find the documentation describing how that part works. If there is no such documentation, then this is an opportunity to add it. If there is such documentation then maybe the wording has to be adjusted. There are many cases where user are not familiar with the jargon used in the project or are for some other reason they are using words different from what you have in the documentation.
For example recently I was trying to figure out as a reader how to get notification when new version is published but I could not find the answer. I sent an email to their support. Within a few hours I got be a link to explanation and a note, that they have updated the response with the keywords I used.
We are all different, we use different words, most of us are not native English speakers, and even the English speakers use different words or different spellings for the same thing depending on their country.
So by improving the documentation you can reduce the frustration of the users. You can reduce the time wasted on support. You can do it pro-actively writing documentation or you can do that in response to question by users.