Hi Studio 6Am
Well there could be many reasons why cron jobs needs to run on the sub-sites in a WordPress Multisite network. The most common and simple reason is this:
In WPMU installations you will have many different flavors of plugins installed and not every plugin will by default run on the primary site or be network enabled. Thus there will be plugin on individual sub-sites that will run plugins that needs to be scheduled to do certain task such as they need to send out emails at a certain time due to various events or lets say a front-end meeting booking for example where the calendar needs to be refreshed and the attendances need to be notified when a meeting is about to start as an example.
The same applies for example to other types of plugins like lets say a security plugin where a malware scanner needs to be scheduled to run at a certain time and date. The same applies. The cron jobs needs to be running.
Now if you the cron jobs does not run on the sub-sites too then the functions of these plugins will simply not work as the crons are never fired up . So that is why it is not sufficient with just having the cron run on only the primary site.
Also you should think about that WordPress itself has a lot of natively scheduled background task and those need to run on the sub sites too in order for WordPress to function properly and if they to do not run there also then the WordPress background tasks will fail to complete their job on the whole network, (except for the main primary website).
I hope this explains it better for you?