It possibly makes the process unnecessarily slow. People tend to choose “round” numbers for their cronjobs. Probably most commonly minute 0 of a given hour for an hourly or daily job. Thus on e.g. 0:00 UTC there might be hundreds of clients running their backups.
I don't have a strict need to run my backups at a fixed point in time (e.g. within the night hours). By not hitting a hotspot I have a better chance of having a larger percentage of the targets bandwidth for my needs (both network as well as disk IO).
The random delay ensures that the job runs at a different point in time every day, with most of these points in time being expected to have a light load. If it accidentally hits a hotspot on one day it will be fine the next.
