UpdateSBSInfo is how any changes you make in the console that affects PXE is sent to the PXE server. We have a doc that details this here:
technical-document-about-ds-71-sp1-architecture-and-data-flow
However, here's the short version:
You send a job to a computer to boot to automation via PXE. We send out 2 tasks - one to the client to reboot, and the other to the Site Server to be ready for the reboot. The 2nd task is the UpdateSBSInfo task, and it will create an SBS file via the deployment agent on the site server. The SBS file is read by the NBS Interface service which hands the data to the NBS Server service (PXE) so that when the client actually reboots, the PXE Server Service actually knows what to do with the client. Same thing happens for any config changes to PXE. So it can run often depending on how much you're doing with deployment solution. VERY often if you're really busy.
TIP: In DS 7.5 it never runs. We no longer use SBS files. Instead, when a client boots, the PXE server service pings the NS to ask what to do, and then responds. It's more efficient. Look for DS 7.5 later this year (fall) as currently scheduled and if nothing happens.