That is not remotely what the philosophy is about. It's about not letting your external circumstances trouble your internal emotions, because they aren't what truly matters. It isn't passive acceptance - Stoics can, and should, try to improve the world around them. They just don't attach their happiness to whether those attempts succeed or fail.
As to why someone would follow the philosophy, it's simple: we all face stressful situations in life (some more stressful than others of course). Why should you let those things rule your emotions? It doesn't help anything to get upset. It just makes you feel worse. It is a pure negative thing in your life. So, you work to try to gain mastery over those feelings so that even when life is hard you can face it more effectively and with greater peace of mind.
What if your successes and failures were due to an external force entirely outside of your control? Would you feel accomplished if things you considered personal achievements were knowingly received due to e.g. influence of being in a higher social and economic class despite lacking the same level of merit as your other colleagues? Should you feel personally defeated if you created a professional project that you couldn't bring to financial viability?
Attaching positive sentiment to the process of personal growth over percentage of success attempts allows you to build a framework of understanding to see where your sphere of influence extends to and where you can focus and continue attempting next to overcome your hurdles. You can obviously be disappointed something bad happens in relation to your efforts, but stepping back without attaching emotions to the situation allows a glimpse at the true impact of your contributions. That's part of my understanding, atleast.
No, not at all. Why would it be pointless? Obviously success is preferred to failure, but ultimately the outcome isn't something I control so I shouldn't rely on it to be happy. The important questions for my own peace of mind are "did I try" and "could I have done better", not "did I succeed".
As to why someone would follow the philosophy, it's simple: we all face stressful situations in life (some more stressful than others of course). Why should you let those things rule your emotions? It doesn't help anything to get upset. It just makes you feel worse. It is a pure negative thing in your life. So, you work to try to gain mastery over those feelings so that even when life is hard you can face it more effectively and with greater peace of mind.