When someone has provided a patch or a pull-request with a proposed change to your project and it has been reviewed, improved and polished according to all the comments and feedback, it will ideally end up in a state where it can and should be merged into a code repository.
Getting this done in a timely manner is an important task for a maintainer and should be done with priority, since that is work done by someone else and therefore once you have merged this piece of good work you might also allow that person to get to work on the next improvement in your project.
A pending merge might not exactly prevent the submitter from starting on that second task in parallel before the first one is handled, but it will work as distraction for that contributor and over time it might also start to get merge conflicts that need to be taken care of. If that has to be done multiple times without a good motivation, it will be demotivating for the author.
Good merge handling shows good project hygiene and vice versa.