There are many ways to increase your movement repertoire besides stretching. I call these methods of movement variability training. I think that much of the reported benefits of many popular movement methods, like pilates and yoga to name a couple, work at the level of movement variability.
Personally, I find showing people how to move into and out of positions that are unfamiliar to them (novel movements) with control and awareness is a great way to improve movement repertoires.
What’s unique about stretching? Sometimes it is done more effectively (by which I mean taken into further or different ranges of movement) when done passively. In general, it is my view that self efficacy should be encouraged whenever possible. So, stretching that requires the assistance of another is not optimal in this regard.
Sometimes we need help, though. And I’ve nothing against borrowing the hands of another to get moving when you’re stuck in a rut. After all, that’s part of what I do for people when necessary.
There are ways of stretching oneself that get around the self efficacy problem. Stretching also often feels “good” and I see nothing wrong with doing something for yourself that is pleasant, even if it is not necessary.
We may at times “need” to improve our movement repertoire and while stretching is one way to achieve this, it is not the only way. My opinion is that the “best” ways of improving movement repertoires are through methods that can be done successfully and safely without assistance once learned and are done without any wild belief systems attached to them. Stretching may fit the bill, and it may not depending on your situation.