On Demand Back up vs Policy Management --> Clients --> Back Up

What is the difference between an On Demand back up vs. going to Policy Management --> Clients tab --> selecting Clients from the tree --> clicking on a client (host) and click on Back Up or right-click on the host and selecting Back Up Client Now? The reason I ask is because a few weeks ago, I ran an On Demand backup for one of our file server and it crashed the file server due to disk space issues. Our backup jobs are ran very often and our file servers tend to fail a lot because of disk space (we're in the process of creating new file servers and consolidating and migrating). After that incident, I was informed by my Senior Admin to perform the manual back up through the Policy Management path rather than the On Demand back up. I didn't get a clear explanation and I couldn't find any explanation in the Avamar Admin guide as well (unless I missed it?). Someone shed some light? Thank you!