That is an excellent question!!!
Basically, Report data syncronization is needed to get data from the production and evaluation databases into the reporting database, so that, whatever reports you want to see, would be available.
So, if you do not have any reporting job's , or if you have all your reporting job's scheduled at night, then there is only report job sync job needed, which would be at night, just before the reporting job runs( Reports are scheduled, i mean)