It might be that the amount of the check shows up as a credit on the receivng bank account immediately, but that does not mean you can already use it (by withdrawing it or transferring it to yet another account). In the case of checks, it’s not unusual for banks to credit the amount of a check subject to a block and to remove that block, and thereby make the funds available, only after the check clears.
For established customers it’s also common bordering on universal for the depositing bank to make the funds available immediately even though the check won’t “clear” for days and is recallable by the issuing bank for up to 60 days.
The depositor bank is simply betting the check will be good. If it isn’t, they’re coming back after the depositor for the money and they’re confident enough they’ll be be able to get it. They’re only out money if both bets go sour. Which doesn’t happen often if they’re good judges of customer honesty.
I was in a meeting with someone at the Federal Reserve recently and they mentioned they are working to get ACH transactions to be instantaneous. So it might be faster soon (I hope).
my credit union wanted to hold the money for a week. I told them I was looking into moving to another credit union. And like magic they released the hold on the money. 
bought a house recently and I had to wire the funds. They no longer take cashiers checks. I think that is true all over. Wire goes through the same day.