Because I look at all these things so that you don’t have to. You’re used to trying to keep track of eyelines, and action continuity, and updating the script changes, and cross-checking your coverage, and all that other stuff. But you’re there to get the frame you want and the performance you want for the shot you want to get that vision you have in your head onto film. God knows it’s hard enough as it is. Me being there frees you from all that other stuff - which is all basically technical and operational, and none of which is creative - and allows you to do what you’re there for. Shoot your film.
I can also shield you from at least some of those constant unwanted questions - I know the script and know what you’re shooting (and sometimes even why) and can answer most of them without you having to break your train of thought.
I’m also a fall guy (sad but true). [NB. Actors and ADs, please stop reading at this point]. Since I’m used to being blamed for everything anyway, you can even use me when you’re not happy with a take to get another one “for continuity” without upsetting your actors. Or getting it past your 1st AD. Hey, camera operators do it all the time when they’re not happy with a take. Or when actors give them a signal that they weren’t happy with a take… (“Sorry - that one was no good, went soft at the end.” “Really - I didn’t see that?” “Yeah. And I jolted the camera by accident in the middle.” “Well that’s ok, we can cut around it.” “And the clip number’s out of sync so there must have been an unintentional button on/off somewhere else.” “Really? Well okay, I guess we’d better do it again then…”)
Oh, and I also do great neck massages.