I'm guessing a good 80% of you Serious Eaters have read Anthony Bourdain's proclamation of love to his squeeze bottles in Kitchen Confidential:
The indispensable object in most chefs' shtick is the simple plastic squeeze bottle, essentially the same objects you see at hot-dog stands loaded with mustard. Mask a bottom of a plate with, say, an emulsified butter sauce, then run a couple of concentric rings of darker sauce—demi-glace, or roast pepper purée—around the plate. Now drag a toothpick through the rings or lines.
And sure, it's a good tool to have if outdated, overwrought plating is your thing. But there are better reasons to own a squeeze bottle than aesthetics. Namely, it'll make you a better cook and better eater.
I think I spend a lot more time thinking about and preparing food than the average person, but even so, convenience defines what I eat and cook nearly as much as good taste. Take salads, for instance. Before squeeze bottles made their appearance in my kitchen, I'd eat them perhaps once or twice a month, and only when I was hosting a dinner party. The hassle of making a fresh batch of vinaigrette just for myself and my wife is simply too big (and forget about using bottled dressing).
These days, I keep a couple of different vinaigrettes ready to go in 12-ounce squeeze bottles in the fridge (any of these will do). Stick your finger over the top, give it a good shake, squirt it onto your greens in a mixing bowl, and boom: lunch is served.
In order to make sure that chunky items like shallots or crushed nuts don't get caught in the tip, sometimes you've got to snip off the tip of the bottle with a paring knife or a good pair of kitchen shears.
As far as condiments go, squeeze bottles are another lifesaver. Sure, you can fill 'em with the standards: mustard, ketchup, and mayo, and of course, you save money by buying those things in bulk instead of in individually packaged squeezy containers. But it's also great for saving money on all kinds of sauces and oils. I buy olive oil, sesame oil, soy sauce, hoisin sauce, oyster sauce, tonkatsu sauce, and Chinkiang vinegar (to name a few) in bulk cans. Store the cans out of the way under the sink or in the closet, and refill your squeeze bottles as needed. It'll make the inside of your refrigerator look all cool, organized, and cheffy as well.
And of course that says nothing of homemade condiments. Mayo is the king, and worlds better than the store-bought stuff, but bbq sauce (I've currently got three flavors), various fancy-pants purées (try cooking a whole orange split in half in simple syrup for 2 hours and pureeing the results in a blender. Eat with duck), homemade plum sauce, tzatziki, pesto, aïoli, sweet miso sauce, chili sauce, garlic and herb oils, creme anglaise—you get the picture. Basically anything that can quickly transform a normal piece of food into something more complex and a little bit fancy. All of them make it into the rotation (if I had the sense to buy even more squeeze bottles, I'd have all of those sauces permanently available).
Want to throw a fancy cocktail party? Squeeze bottles are your friend. Fill a big one with simple syrup, smaller ones with fresh squeezed citrus juice or flavored syrups. You'll be cleaner, neater, and more efficient, cutting the time it takes to make each cocktail by a not insignificant degree. Your guests will thank you and marvel at how pro you look.
And yeah, like Tony says, you can use them to make your plates all frou-frou if you desire.
As far as buying them goes, no need to get fancy. I have a couple dozen that I picked up from a Chinese restaurant supply store. Amazon sells them for a couple bucks apiece. Buy a half dozen, see if they don't change your life for the better.
Pro Tip: For easy sauce transport, take the lid off the squeeze bottles, cover the hole with a double layer of plastic wrap, screw the lid back on. You sauce will stay 100% drip-free until you get to your destination.