I think my mother used to use it for roasting potatoes in although I think goose fat is more popular for that these days.
My Dad and brother used to spread the dregs of it (with the crispy bits of beef) onto a slice of toast.
From Wikipedia:
"Pork or beef dripping can be served cold, spread on bread and sprinkled with salt and pepper (bread and dripping). If the flavourful brown sediment and stock from the roast has settled to the bottom of the dripping and coloured it brown, then in parts of Yorkshire it is known colloquially as a "mucky fat" sandwich".
If its the filtered stuff in a jar, then it wont have a lot of flavour and would principally be used for roasting spuds and other veg or doing yorkshires. I can't imagine that it would flavour a sauce or stir fry.
You use it to roast things. Produces a more artery clogging but tasty alternative to all those healthy oils. As I recall my mother used to use little else in the oven. Not used it for years & years though. Ah those were the days.
I used to ask for a slice of beef fat when buying my weekend joint just to do the roast potatoes and Yorkshire puds. No taste better IMO. Waitrose sells frozen roast spuds cooked in beef dripping. Lovely.