This traps them with a poisoned Pokmon, disrupting their plans and stopping any backup strategy they may have been preparing on the bench
It is AI's turn to play
(Of course this effect is very small- it would take something like 20 heads in a row before one should start thinking the coin is biased, unless you were given it by an exceptionally untrustworthy friend.) The same principle applies to Magic
And then you can also do the dreaded bounce, block, bounce and stop an attack (You turn it from an aura to a dork, block, and then tap to make it an aura before damage is dealt, and the attacker is still blocked,)
Interestingly, when the Legendary Collection was released years later, the Dark Blastoise artwork was chosen over the Base Set design for the reprint, further cementing its legacy as a fan favorite