Hi,
The flavor of what is fried in the oil will impart a flavor into the base.
In the kitchen at the restaurant I remember one day tasting the base gravy I was preparing and noted that it had a strange/different background taste to it.
The base this day was start off with oil from the big thick metal wok thing that sits on the burner and is used to fry samosas, bhajis and all sorts of things, no chips though! It was only then that I realized we had fried tandoori chicken wings in the oil before hand as the chef was making a tandoori wing biryani for the buffet and that was what was coming through in the base, at that stage anyway.
The bases aren't always started with used oil, not sure why the chef said to use that oil that day, perhaps they were low on oil. I asked him about it once and he said sometimes yes and it does add a little flavor, but it is not necessary nor done all the time.
Sometimes, it's just handy, and other times it's perhaps because they are running low, but certainly not critical ingredient. Each week the oil is dumped at least once and sometimes twice, it's not kept too make gravies with.
Cheers,
Mark