Being a writer means you’re the boss of your own work. This can have benefits as well as downsides. It’s nice when (at least at first) nobody is looking over your shoulder, but it also means you’re the only person who can assess your own progress. And just like you have evaluations at ‘normal’ jobs, writing should be no different. With anything important, it’s vital that you know your strengths and weaknesses so you can use and work on them, respectively. How else do you improve?
Other than the obvious reason of self-improvement, tracking your progress is also handy if you want to discover your habits. Perhaps there are certain days that you are more creative than others. Maybe you find that you write more in the evening, or on Wednesdays, or when the moon is full and there’s been a blood sacrifice. Either way, use that. Habits can be learned, so perhaps you want to force yourself to write only on weekends when you’re more productive instead of forcing a few hundred words over the week. No need to torture yourself. Save that for your characters.
Personally, I track my progress in my bullet journal. It helps to see on paper what I’m good at and what needs improving. I collect this based on my own evaluation of my writing but also based on feedback from beta readers. I focus on one or two things I want to improve first. It motivates me to see what I can cross off because I’ve gotten better. Every story is different and with something like writing it’s difficult to see progress because you’re so focused on the novel. Every story brings you closer to being a better writer and that in itself is rewarding because each book deserves a good author.
Looking back at where you’ve come from is a nice kick in the butt for when insecurity rears its ugly head. No matter what you do, I believe that everybody has bad days and it’s good to see for yourself what obstacles you’ve overcome. Keep writing. Keep improving. Keep having fun.