Max Motor Speed is used to protect the motor against being driven over it's maximum speed. The units for this are RPM.
Max Profile Velocity is given in same units as the profile velocity and applies only to profile position mode.
(Source CiA DSP 402 V2.0)
While the standard doesn't expand on these, the way I see it is that the first number is something you set up based on the motor's data sheet to prevent damaging the motor. The units are set up that way.
The second number is an application specific limit. You want to protect yourself against moves that can be damaging in your application and you use the same units used for position profiles.
Having two parameters allows for running the motor in velocity mode in higher speeds than you allow during positioning modes. While that may not seem important I've actually worked on an application where a plate was loaded on a drum and the drum then spun up to considerable speed. In this application the loading portion used positioning mode and needed to be slower than the velocity mode used later so this separation of limits could have come in handy. In general having those sorts of limits built-in reduces the chance of damaging the machine you're working on.