Dude change your clocks something like daylight savings then you can use it several times at home instead
You know like a time machine, then you can fix your stuff before you get to the dive site, which is for diving
Not sure that I follow (but I do like the time machine reference), so let me explain the water line thing:
I drive to the dive site, spend some 20 minutes for parking, then a trek of 300 meters with all the gear to the water line. It is scorching summer, 45 Celsius and you sweat like hell and want to enter the dive. Of course, since you do not want to discover that you forgot, say, the mask in the car (been there done that) you meticulously check all your gear.
Then your buddy arrives, you want to open his valve and the knob moves freely. Now, you can either go back to car, drive to dive club and ask to replace the cylinder or the knob. Then search for parking again etc. This can take an hour if not more.
Have a valve tool in the car? Nice, still a trek back and forth in the scorching sun.
Or take out the tool from pocket, and two minutes later can be on your way to dive.
So, lots of minor repairs can be done in the water line no need to run back to car or to dive shop. From stuff like replacing O-ring, tighten a loose hose, mouthpiece or whatever. No time machine needed, although I agree with you that it is better to have everything tip-top before you get to the water, but like anything in life **** can happenz even to the most meticulous OCD divers who triple check everything. Therefore, it is good to have a small multi tool, just in case even if you hope to never have to use it.