It matters because employers are required to pay holiday pay on regular overtime hours but not on ad hoc hours.
I work for a school trust doing self contained projects type work. I'm employed through payroll for a certain number of hours per term, approx equivalent to 2 weeks full time, which I do flexibly as and when the work is required.
I submit time sheets termly and am paid overtime (excess hours) for anything over my contracted hours.
Every term I'm claiming at least another 60 hours, but I'm told it doesn't meet the "regular" definition because it's not regular weekly hours, but I'm not employed that way.