There are a lot of different opinions about this and I got asked this by clients a lot … I don't have a short answer (like, do I ever?) but here are my thoughts about it...
First of all, for me, yoga can be seen as both, but I see it more as a tool to maintain your body AND your mind healthy. To tune in. To find out what need to be strenghten and what needs to be released or let go of, both in your physical body, mind and in your life. Yoga can teach you patience andsticking to Things by going through a certain form of discomfort using your breathe. Knowing how to use my breathe has been helpful for me in so many situations, be it performing a tough run or workout, handling turbulences on an airplane or during stressful situations at work. It didn't really burn any fat or tone up my muscles, but here it is and it's great and I think I wouldn't be able to to this without yoga.
Also learning to observe and take care of my body helped me a lot to make healthier food choices or to take care of myself when I was sad or tired. I learned to "check in" and notice how my energy is at every moment. Acknowledging this and making my decisions based on that helped me to take better decisions and to be more focused and productive in the end.
Over time, I became very sensitive about what kind of energy other people around me had and if it is doing any of us any good. and surprise - I attracted the right people and the right situations way more when I practiced this "checking in", which is a big part of Yoga. And yes, performing advanced poses and inversions is a lot of fun. The physical aspect of yoga is what brought me there (but did'nt necessarily keep me there). And performing advanced poses (or learning the techniques and building the strenght that brings you there) will work your body in amazing ways for sure.
With that said, there are clearly easier ways and workouts to shape your body if this is all you want. Strenght training has been around for a reason and so has cardio training. I don't say any of them is better than the other. For me Yoga is my base, I would always start with that and it helps me to improve with all the other things in sports and in life too.
And last but not least, if you are not a "sporty" Person or running a marathon and having abs means really little to you, let me tell you that definitely don't need to be super strong and flexible to get all the benefits from yoga!
Speaking for me, yoga is checking on my body every day and I think that the saying "we don't practice yoga to get better at yoga, we practice yoga to get better at life" is really true. and it works for every life and body. If you can breathe you can do yoga! If you want to build strenght and tone you body, adding bodyweight exercises will do a great job. Stretching at a yoga studio 2x a week won't bring you there and we should all be honest with that ! Almost every "fitspo" yogi does other training too or has been a gymnast or dancer. I mean, if you wanna run a marathon, you definitely should perform some cardio specific training too and everyone would agree on that.
With that said, I truly believe adding yoga to any of these workouts will make a GREAT difference ! We often Forget that it is mostly our minds that we have to convince … and Yoga will definitely train it too !