As someone who worked on an Open Source project for years with no compensation, adding sustainable funding to it was very motivating. Money is a very weird thing, but getting money for the continued existence of your work is fantastic validation and support. It's easy to work on things for free, but how do you know that people value them if they simply use them for free. Attaching money to a pursuit that was previous…
More subtly, I find that having monetization as a goal of a side project can creep in in a lot of ways that aren't really beneficial to the project. Take even just writing something. Sometimes I'm frustrated by the information available online on a subject, and want to spend a bit on writing something to set the record straight. Sometimes this makes sense to publish myself. Other times it makes sense to write or improve a Wikipedia article. This is an interesting tradeoff, because it's basically trading likely many more views (therefore likely more impact) if I write it on Wikipedia, versus more control and authorial credit if I post it myself. From a monetization perspective, writing on Wikipedia is rarely the optimal choice; but from the perspective of getting the information out there, it sometimes is.