Why is the speed of the earths plate measured in centimeter per year instead of in meter per second?

Just to get a more convenient number.

1 centimeter per year is the same as 0.000000000317 meter per second.

Measured in centimeters per year, you get a number that's much easier

to write down, remember, and tell other people about.

Exactly the same reason that we describe the speed of a snail in

feet per minute, not miles per hour.