I am a big believer in assessments. In every aspect of life. They can give you an objective reading of where you stand. Sometimes it tells you what you like to hear, and other times it tells you what you don't. That's kinda the whole point.
I use assessments a lot. Specific, general, subjective and objective. It depends on what the situation or individual calls for. Usually, the more specific the goal, the more specific the assessment. .
I use assessments for strength, movement, mobility, flexibility, body composition and even personality. This tells me what I should prioritise when working with someone, and the best way to go about prioritising it.
As a coach, this seems like the obvious things to do. But what about when training by yourself? IMO it should still be done if you want to get the most out of the time and effort you are investing. Optimally you would pay someone, or if not you can figure out a way to assess where you're at.
See as humans, we gravitate towards what we are good at. Flexible people love to stretch when they most likely should do the opposite. Really strong people love to lift. Strong benchers love to bench while strong pullers love pull-ups.
Sport specificity aside, if general fitness/wellbeing and longevity is the goal, then a balanced approach is gonna be optimal. Assess to find your weakest link, then work on it until its not your weakest link anymore, and then reassess.