My 1980's Walther PPK had no slide device, and to lock it back, you needed an empty mag. That was way back then, the design may have changed.
It does help with maintenance and troubleshooting to be able to lock the slide back easily (say you have a mis-feed, or some malfunction, where you actually have to reach inside). It is a PIA if you can't lock the slide back.
Most AK rifles do not have a bolt lock, unless you notch the safety lever, or modify the mag follower, the bolt stays closed.
On the range, you can stick a high visibility item into the chamber, that is visible to all around you, indicating the slide is out of battery (pencil, yellow cable, highlighter marker). You may be asked why your slide does not lock back
. As long as no one physically touches or handles the gun when the range is cold -- it does not matter if the slide is open or closed.
Liability and common sense : This will be used against you in any legal process you are involved in.