With a candle, you choose when. With a diffuser, you choose where — and that single decision does most of the work.
Where they earn their place
Entrance halls, first of all. A diffuser is running before anyone arrives, which is exactly what you want from the room that sets the tone for a house. Bathrooms and cloakrooms, where you want something present without wanting a flame. Studies and bedrooms, where a low constant is better than an event.
Where they're wasted
Directly beside an open window or a fan. Air movement speeds evaporation up dramatically — the scent leaves the room as fast as it enters it, and you'll be through the bottle in weeks rather than months, with less to show for it.
Also: very large open rooms, if you're expecting a candle's presence. One 200ml bottle holds a modest space well. It will not perfume an entire open-plan floor, and no amount of extra reeds will change that.
A word on surfaces
Fragrance oil will mark wood, paint and some stone if it's spilled or if drips run down the neck. Stand it on a coaster, a tray, or a ceramic dish — not directly on something you'd be upset to damage. When you flip the reeds, do it over a sink or with a cloth underneath, then wash your hands.
Height helps
Scent falls as it disperses. A diffuser at chest height on a console or shelf reaches a room better than one on the floor. Not a rule, just physics being mildly helpful.