the point is you are still wasting your time and your life on stuff that has no practical value for 95% of developers. You are spending a huge chunk of time learning skills that have no use beyond the interview itself.
Also, even if training for algorithms questions was purely a matter of signaling with no positive externalities - it's actually a much faster and easier process than getting another degree, which is a common alternative.
TL;DR If you put in the time, it can absolutely change your trajectory in ways that you might like a lot. If you don't, how will you know?