Comma help before the word 'with'

He also had the support of the crowd, with his hometown of Philadelphia being just up the road.

I never know when to use a comma before 'with' or not. Can anyone clarify the difference please?

[Tdol]

The part after the comma gives us some extra information about the reasons for the support of the crowd. You could cut it out if you liked and the sentence would still be grammatically complete, which wouldn't work with something like I'm going with them. The comma halps separate the main sentence from the extra information.