Zan-shin: literal translation is "remaining mind" which means "undivided alertness".
No cut will be considered complete unless a correct Zanshin is implemented. Always.
Zanshin is not a physical attaching technique but it's a way to exhibit the move required in real sword fighting.
It does not require physical strength hence omitting Zanshin / doing it wrong is just a proof that you have not understood the basic kendo protocol, or even the meaning of kendo.
In short, failing to do the correct Zanshin makes your kendo superficial.
Zanshin, originally, was meant to be done towards the person who fell down by receiving a fatal injury from you.
The purpose of Zanshin is not to be complacent about your victory and keep alerted even after your opponent has fall down. (Because they might just be faking it.)
So, Zanshin in olden days was often done towards a dead body, but in case the opponent is faking their death, you still stand a chance with a correct Zanshin.
Failing to do Zanshin could lead you to your demise. If you can't be asked to follow such key protocol, you might just not bother kendo altogether, because there are lots of other protocols that seem pointless unless you understood its history.
You don't need to know everything but when something is required for a correct cut like a Zanshin, you'd better do it properly. Doesn't matter if it's in a match or basic practice, when you cut, do Zanshin, always.
It's more important to do Zanshin in non-competitive set up because unless you makes it a habit, it won't come up during a match, and no Zanshin almost inevitably means no Ippon in a match. No matter how deadly your actual cut is.
Once you cut and gone through, turn around asap and back to cyuudan with a readiness to launch another strike.
Only then, it counts as the end of a cut sequence.