Sometimes ATC want you to descend early and you don't want to descend yet so you use VS to keep a low rate of decent since your not on a profile. Sometimes you are below an ideal profile so you use VS to keep a slow descent until you reach the ideal profile.
Sometimes ATC want a specific ROD or ROC.
Sometimes you only want to climb a thousand feet and a full power climb isn't warranted since you don't want to cause TCAS conflicts or discomfort to passengers….
There are many many reasons
