In the old days, removing and replacing the seals could be done by prying out the old one with a flat blade screwdriver and pushing in a new one. The latest generation of rear main seals are a very tight press fit and require a tool of some sort to remove and install. Prying out with a screwdriver doesn't work well. It levers the crank forward, risking having the inside crank shim fall off it's pins.
Used this tool a few times and it works very well. Well worth the price.