So, unless I’m missing another good reason, the only good reason for this rule is to give the players in the game an option to confirm their suspicions if they think there might be collusion going on?
Essentially, yes. So the answer to your original question:
Someone looked up the rules and said that the player that calls on the river is entitled to see the opponents cards.
... is No, but actually Yes, but actually No. But actually Maybe.
A player doesn't
have to show their hand; they are allowed to discard it rather than contest the pot. They only have to show their hand if they're trying to claim the pot, and they don't even need to do that if everyone else discards first.
BUT ACTUALLY everyone who was dealt into the hand is entitled to see any hand that went to the showdown. The rules here are slightly fiddly wrt which hands then become live or dead depending on who asked to see which hands, and the rules are slightly different between RROP (cash games) and TDA (tournaments). But the basic idea is still that hands discarded at the showdown can still be asked to be shown.
BUT ACTUALLY this is not a strict entitlement! Asking to see discarded hands is a privilege, and that privilege can be revoked if abused.
... but what constitutes "abuse" ? There are at least two different schools of thought here. The first is that the privilege exists
in order to prevent collusion, and that the privilege should only be invoked in cases where collusion is suspected. Using the privilege simply because you want to know what cards someone takes to the showdown so as to better learn their betting patterns constitutes an abuse of the privilege.
HOWEVER the second school of thought is that the privilege exists
exactly so that you can see what cards someone takes to the showdown! Having "paid to see their cards" you are in fact entitled to see those cards, even if they discard them and yield the pot. The reason one might not is either courtesy, apathy, or expediency - most would rather keep the game flowing to the next hand rather than turn over every hand at every showdown. But one could still ask, even if possible collusion isn't a factor. Abuse of the privilege constitutes asking to see discarded hands
every hand, because doing so slows down the game, and thus if the host thinks you're using your privilege "too much" then it might be revoked.
RROP is silent on which of these two interpretations is correct. Most go with the first but some go with the second, as you can see in this very thread.
SO ACTUALLY the answer is Maybe, because it's up to the host to decide whether you can demand to see the discarded hands only to prevent collusion or instead any time you like as long as it's not too often.