Quarantine is most important when getting new fish that you plan to add to your main display tanks. Buying fish from a store for example, putting them straight into your main tanks is the most common way people end up spreading illnesses/parasites, and risk losing their whole stock of cherished and/or potentially very expensive fish.
Keeping new fish (often bought as a group) in quarantine for 2-4 weeks can save a lot of heartbreak and expense. Fish meds aren't cheap, and dosing a ten gallon QT would have saved me having to medicate for both kinds of worms in all four of my larger tanks! There are people with hundreds of gallons worth of aquariums, and fish worth hundreds of dollars each. You bet that they QT new fish until absolutely sure that they're disease free.
Given how common things like worms and ick are in fish stores and farms, a QT is always sensible.
