Pre-registration plans (PAP) rightly become more common (they are still not common enough yet, I think), but here’s a reason to write up a PAP that I have never seen mentioned before: Pre-registration plans can be immensely useful for yourself!
So, you have come up with a clever analysis, and writing the PAP has helped sharpen your mind what exactly you are looking for. You then collect your data, finish off another project, and … what was it exactly I was going to do with these data? Did I need to recode the predictor variable? etc.? Yes it happens, and a pre-analysis plan would be an ideal reminder to get back into the project: PAP can be like a good lab journal or good documentation of the data and analysis we do — a reminder to our future selves.