Julienne applications
I don't know if I'd consider the cut I use for stir fries to be a jullienne, as I tend to make 'em a little larger, as I like a little bite left in my vegetables.  (I consider julienne to be at most 1/10 of an inch / 2.5mm across ... I cut my carrots for stir fries at maybe 5mm across)

I tend to julienne when adding vegetables to cole slaw-like salads  (or even apples and other firm-fleshed fruits).

If you like really crispy fried potatoes, you can julienne the potato before frying to make 'matchstick potatoes'.

Many professional chefs will have a mandoline, and so if they're preparing large amounts of something, they'll use that, rather than a knife.  (and you can get longer strips that way ... the full way down a large zucchini or long cucumber, which would be difficult with even a long knife).

Julienne is also just a step before finely diced items, so technically, I'll pass through that phase when cutting up carrots for mirepoix.

... all that being said -- you're probably most likely to encounter the term when dealing with fancier french cooking or "gourmet" type recipes where you're trying to impress a dinner party. "Home" cookbooks are less likely to get into complicated knife tasks.