There are some instances when the randomization may not sound right on your pattern. This may be down to the instrument being used not working well with the velocity randomization or the amount of randomization being too much or too little.  It is also worth noting that some patterns, such as hi-hats, work well with more predictable velocity. This is what helps give them a sense of groove and is how real-world drummers play. So in these instances, it is worth manually applying a velocity pattern.