TL;DR: Is there a technical reason for ever turning off the Redstone "clock" in this mob farm design (single-player world)?
In another Arqade question I posted earlier, I explained that I'm a Minecraft n00b trying to build a mob farm in Survival Mode for my compound following the instructions I found in the Basic AFK Mob Farm – Minecraft 1.14 – Basic Builds video from BlendsCraftTV. There are several questions I have about the mob farm design, but my next question for those of you experienced in Minecraft is regarding the Redstone "clock" used to activate the "flushing" system (starting at about 6:44 in the video). Turning it on with the lever starts the Redstone repeaters ticking away, which triggers the Observer, which then triggers the Dispenser to release the water in timed increments, thus flushing any mobs on the "spawning floor" into the already flowing water, which then drops them down to the killing floor. I understand those mechanics but, what I don't understand is why you would ever turn the Redstone clock off.
I mean, according to the instructions, you turn it on just before you go AFK, then turn it off before going down to collect your goodies. That means, to me, that the spawning is taking place while the flushing system is active, so the cycle should continue indefinitely as long as the Redstone clock is running, which leads me to ask, why would I ever want/need to turn off the Redstone clock?
The only reason I can think of would be to allow a larger number of mobs to collect in the spawning chamber so that the next time you start the clock you're able to harvest a few more resources, but I honestly can't think of any "good" reason to turn the clock off or any "significant" detrimental effects of leaving it running all the time.
Best Answer
TL;DR:
You should be fine not doing so but it is a good habit to shut off your contraptions and farms.
To summarize the answers you already got in the comments, and give my view on top:
Since you are building this in your single player world it would not be a big problem to have it running all the time, but what if you ever want to open your world to LAN or upload your world to a server because you grew attached to is but want your friends to join in?
That is where you need to start thinking about everybody elses experience and the capacity of the Server you are playing on. Calculating the flow of water and the lighting updates caused by your redstone curcuits eats away on the performance little by little.
In addition to that you should always consider the possibility of you contraptions breaking on unloading if they partially unload while running (build the thing between chunks and on borders). Although I do not think I have encountered a bug that would break your design yet.
In the end it is a good habit to include a switch to turn anything of to make the Server as lagless as possible.