Once players have collected one of each of these Pokmon the 3 Star Mew card will unlock
For example, the Professors Research card allows you to draw two additional cards on your turn whilst a Gym Leader card, such as Sabrina, allows you to swap out your opponents active Pokmon for one on their bench
You can also do things like swing as a creature with myriad, then transform into a more interesting creature that you actually want to copy, or you can use the counters to become versions of creatures with way more power than they ever expected
However, this comes at the small price of not being able to tribute it, or use it as a material for Fusion, Synchro, or XYZ summons
Further Reading: Hidden Gems for Hashaton, Scarab's Fist Underplayed Draw Spells for Commander
After all, its not easy to keep track of everything when a new product is released every 4.4 days on average