I hope this at least solves the people 'gps bombing' certain popular segments with fake times. I use all three of those apps you mentioned and generally here is how I frame them:

TrainingPeaks - analytics, record keeping, tracking improvement, interacting with my running coach, planning build up to events, making notes to self about training. (Training and planning)

Garmin Connect - where I record, check data, and handle posting. Since buying a watch, four button clicks on my wrist has music playing and my scheduled workout timed out and just beeps at me when I need to speed up / slowdown. It probably gets me out the door 5 minutes faster than using my phone and strava which isn't nothing (Tracking).

Strava - fun way for me to interact with others. Minimal useful information, unless I'm being silly competitive against myself or others. (read: social)

It takes zero effort to have Garmin and my watch sync with both TP and GC, while if I record on Strava on my phone I have to download a file and upload it to the other two. the GC push to strava has significantly reduced my engagement with strava from once a day to once or twice a week where I bulk comment / thumbs up others workouts. If I didn't have running friends who were in other states or training peaks had social elements I would probably slowly fade to zero usage on Strava.