I use a mandoline to slice vegetables more often than most home cooks—maybe five to six times a week—and yet I know that I use a mandoline far less often than I should.
Mostly that's because I have a bit of a knife-skill hang-up. I believe, like all skills, that you only improve with a ton of practice, and so rather than pull out the mandoline to shave some white onions for pho, I'll use a knife. But if convenience, efficiency, and speed were a priority, I'd just use the mandoline: in the time it takes for me turn half an onion into a pile of irregular but fairly thin slices, I could shave down a whole onion into a mound of uniformly razor-thin slices.
That efficiency is particularly useful for larger cooking projects that require a lot of thinly sliced vegetables, like, for example, chili crisp or Thai-style fried shallots. There is a functional argument for using a mandoline, too: the more uniform the slices of shallots (or whatever vegetable you're slicing), the more evenly they will cook.
I understand all that very well, but I use my mandoline mostly for a different task. Namely, turning hardy root vegetables into tender ribbons for salads.
The best way to incorporate carrots or radishes into a salad is to slice them extremely thin, but doing so with a knife is a tricky proposition: you have to square one side off to form a steady base, then you have to rely on a (very) sharp knife and a (very) steady hand to shave off slices, most of which will be a little too thick. With a mandoline, all you have to do is set the thickness and start sliding the vegetable across its surface.
But there are other vegetables that really benefit from being mandolined, and chief among them is kohlrabi. I never really knew what to do with kohlrabi until I was talking with Daniel about broccoli stems. At one point, I said I was buying heads of broccoli just to peel the stems and shave them on my mandoline for salad (dressed with just good olive oil, flaky salt, a little lemon or red wine vinegar, black pepper, and a fistful of Parmigiano-Reggiano grated on a Microplane, so it's all fluffy), and Daniel said, "That's crazy! Just buy kohlrabi!" And he was right: I can eat a pound of kohlrabi treated in a similar fashion: Peeled, quartered, then shaved as thinly as possible on a mandoline to produce fat ribbons that are perfect for trapping dressing within their folds.
Since I use a Benriner at home, I don't ever use the mandoline for julienning vegetables (as Daniel notes in his review, those attachments for the Bennie are basically useless), but I do use it to create uniform planks of whatever vegetable I want to julienne so I can eat root vegetable slaws. It can be as simple as a pile of carrot sticks dressed with mayo and mustard or a mix of carrot, celery root, kohlrabi, and watermelon radish tossed with a tight vinaigrette.
I suppose one can't write about a mandoline without talking about finger safety, and while I've nicked a fingertip or two when using my Bennie in unguarded moments, you won't have to worry about any risk of injury so long as you commit to using a (clean) kitchen towel (what can't they do?!?) to push the vegetables onto the mandoline blade every single time.
Of course, if you're a fancy home cook, the mandoline is great for filigree-thin shavings of vegetables for garnishing, but I don't really look at it as a fancy tool for fancy people; I look at it as an assistant designed to make some dishes, like a silky rich potato gratin, easier to make, and a tool to transform the humblest of vegetables into daily, delicious fare that just happens to have a touch of elegance about it.