After a decade of having coaches always telling me what to do nearly every day, I wrote down plans when first graduating college. It just made it easier for me to look at the calendar and say tomorrow is a tempo run or tomorrow is a hill workout or whatever. Then, I could just do it, just like I did when being coached.
Over the years, I've gone more the other direction until now, I sometimes don't know what the workout will be until I'm mostly through my warmup. Sometimes, I don't even know whether or not I'll do a workout until I'm a mile or two into a run.
I think there are advantages to both. If you need something to hold you accountable to get in your workouts, having the written plan can be great for that. However, if you can keep yourself accountable without the written plan, it's human nature to be less flexible when you have something written down in front of you. Sometimes, that lack of flexibility can be a bad thing and just having an idea of what you want to do can make it easier to be flexible when necessary.