It is really helpful if you are web developer. For instance, if you need to alter some layout that was made some other units, it is good to know how the units relate to one another.
yes, sure, but this is not about that, all CSS unit have the fixed relations clearly defined. Angular pixels are just about relative scale between the size on the screen and the real-world size (whatever that would be, because a print size is again relative to the chosen dpi, so it's not an absolute measure)
