Tracking every hour of your day?

I recently started to track my time in my calendar and found it a good way to stay more focused. Instead of a negative habit (stop wasting time on reddit) I found it to be a positive habit (spend time as best you can). I like your idea, and I hope my own experience will help you:

Why would you track hours?
I like seeing how I spent my time. At the end of each week (rather, at the beginning of each week) I review my week and draw some conclusions of what I could be doing better.

How could it help you be more productive?
Seeing how I spent my time makes wasted time jump out at me.

Have you tried but stopped? Why?
I tried a bunch of times before, but stopped because I only did it as a chore, not as a tool for personal growth.

What would you use to tracked hours?
I usually use Google Calendar.

Do you track hours, 30 minutes or each minute (?!) or something else (like a diary)?
I track time in chunks of 15 minutes. For instance, my browsing of Indie Hackers just now might take 5 minutes, looking at some Facebook groups another 10. I'd track this as "Browsing" for 15 minutes.

What categories do you track?
I have a few ongoing projects that each have their own category and color coding. Personal stuff in another category, my personal projects in yet another, and a final one for family time.

Should it be a mobile app or both?
I would use this as an easy way to input data and have it sync with Google Calendar. Unless it has a spectacular UI, in which case I would use that as my main calendar if it had GCal sync.

Should it be a future looking app appose to just retro active?
Both! I usually set up fixed appointments ahead of time, and track my time spent as I work/live.

Hope this helps! :)