Recently, Jay Honeck posted:

The other day, after launching from Iowa City, Cedar Rapids Approach
(CID) gave me my squawk code, as usual.

[...]

Before I could read it back to the CID controller, he had moved on to
another plane.
[...]

So why do we read it back?

*Does* everyone read it back?

I "read it back" before I change the transponder, just as I do for any
other controller instructions. If I enter the wrong number, they'll let me
know, if only to say "no radar contact".

Neil