Throw pillows get dismissed as filler, the thing you toss on a sofa to make it look finished for a listing photo, and I get why. Most of them are thin, flat and decorative in the worst sense. But a good one changes how a room feels to actually sit in, and that's not nothing.
They work hardest in rooms where people actually sprawl, a den, a reading chair, a bed you flop onto at the end of the day, rather than a formal living room nobody uses. A couple of large ones on a sofa make it feel finished without trying too hard, and a pile of smaller ones on a bed earns its keep through a long winter when you want something to burrow into. In a sunroom or a spot that gets strong afternoon light, go for cover fabrics that won't fade out in a season, and think about the climate you're in generally. A down insert feels incredible in a cooler house and turns into a hot, flat mess somewhere humid.
Before you buy, check the insert separately from the cover if you can, because a cheap flat pad inside a beautiful cover will look sad within a month. Press on it. It should spring back, not stay dented. Check the zipper is on the back seam and not along the bottom where you'll feel it every time you lean back, and if the cover doesn't come off at all, know what you're signing up for down the line.
Where they go wrong is piling too many patterns or textures into a small space, a busy cushion against a busy rug against a patterned chair just reads as noise, not personality. Keep one loud piece and let the rest be plain, or it starts looking like it was assembled rather than chosen. They also do badly in rooms with pets that treat them as chew toys or a kitchen nook where they'll just collect crumbs and grease, better suited to rooms without a direct line to the food.
Covers with removable cases and a decent fill hold up well, outlasting a couch cushion underneath them in some cases, since you're not sitting directly on them all day. Feather and down inserts need fluffing regularly or they go flat and lumpy in patches, while polyester fill holds its shape longer but never gets quite as soft. Over a year of actual use, covers fade near windows, corners wear soft, and any light fabric picks up marks faster than you'd like, so a removable, washable cover earns back its cost many times over.
Check the care label before anything spills, because some covers are dry clean only and some can go straight in a cold wash, and you want to know which before the wine, not after.