What is the purpose of a keel roller on a boat trailer?

How does it function?

Originally they had a deeper V in the keel rollers and helped guide the boat on the trailer. This was years ago when most boats were smaller, had deep V hulls, trailers tilted at the tongue, and boats were winched on. As boat design changed ( less pronounced keels )the blocks that run along side the keel were added to help guide the boat straight when going from one roller to the next. These added blocks also allowed the boat to be driven onto the trailer.
On today's trailers the keel rollers help keep the boat keel from hitting the cross members when loading and unloading. The angle of the ramps and how for into the water the trailer is determines when the boat touches a roller.
If all ramps were the ideal angle to the water, boats could be loaded without using rollers, and need only keel blocks. The steeper ramp angles may help unload boats, but when loading the front of the boat keel will go between the blocks and hit the crossmember unless it has a roller.
I have added a keel roller on the very front crossmember of my trailer because at some steep ramps the keel would hit just as the bow of the boat came off of the bow roller ( or V block )

BTM