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 to 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.
:max_bytes(150000):strip_icc()/__opt__aboutcom__coeus__resources__content_migration__serious_eats__seriouseats.com__2018__03__20180322-chili-crisp-vicky-wasik-collage2-1500x1125-9460474f20b849e1a537a3e315bd12b4.jpg)
Serious Eats / Vicky Wasik
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.
:max_bytes(150000):strip_icc()/__opt__aboutcom__coeus__resources__content_migration__serious_eats__seriouseats.com__images__2017__04__20170420-what-to-do-with-broccoli-stems-vicky-wasik-11-1500x1125-47ca09198ad549399cdff70535177ab4.jpg)
Serious Eats / Vicky Wasik
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.