In short: get things out of developers' way.

In more detail:

1. All typical project management stuff shouldn't take anywhere close to full time job in such a small team, so PM should deal with all standard PM tasks: formal communication (especially with customer if you have one), dealing with stakeholders, organizing work on general level (I don't say they should tell developers how to code), managing requirements, dealing with priority changes etc.

2. Additionally there are tasks which PM can deal with and usually do it well. I'd say that PMs are very good testers on average. I mean all sorts of functional tests here. That's because they usually know very well what the project is all about and how specific features should be developed to fulfill client's requirements well. What they lack in knowledge about QA (if they do) they make up for with knowledge about project.

3. Another thing like that is documentation. A type of documentation really depends on a project but most likely you need either something for administrators, users or both. When I think about documentation I mean things like on-line helps too.

4. User support. Again, level of support you need depends on a project but if you build some self-hosted web-app addressed directly for end-users you'll have some questions, bug submissions and feature requests altogether somewhere in your support inbox. Someone has to deal with it and a PM is way better candidate than a developer. If you work on a custom project for big customer there will also be some of these but then it is usually more formalized and goes to standard project management tasks.

5. Team management. It is not unusual in this kind of teams that PM is also a line manager. However it shouldn't be related with them being a PM but rather with having best managerial skills in the team.

6. Sale/marketing support. With small team it is likely you don't have a big team of salespeople trying to push your stuff to target market and sale process either requires some subject matter experts talking about the project (selling project to companies) or marketing effort to reach end-users with your app (selling application directly to end-users). In small teams most of the time PM is a best candidate to do so.

Note, it all much depends on who do you have on board. I haven't mentioned things like working on best development practices (although I personally do this too) since you stated you can take care of this stuff. Anyway it all comes down to the people. PM in general should cover all gaps you have in the team, either with their own hands if they have enough time or by getting more people to the team.