It comes down to what's actually in the tin. Premium paint carries a higher pigment load — more colour per litre — plus better-quality binders (the resin that holds it all together and sticks it to the wall) and finer grinds that give a smoother, more even finish. Cheap paint pads out the volume with water, fillers and chalky extenders, which is why you end up doing three coats to get the coverage a good paint gives you in two.
Where you really see it is depth and how the colour behaves in changing light. Brands like Farrow & Ball, Little Greene and Mylands load their colours with multiple pigments rather than a single black to knock things back — that's why a F&B grey like Cornforth White shifts beautifully through the day instead of going flat and dead. Little Greene and Paint & Paper Library do the same trick with their tonal ranges.
But here's the honest bit: premium isn't always the right call. For a ceiling, a rental, or a room you'll repaint in two years, Dulux trade emulsion or Crown will do a cracking job for a fraction of the cost. Where premium earns its keep is feature walls, period rooms, anywhere the colour itself is the point, and surfaces that take a hammering — kitchens, hallways, woodwork.
Washability matters too. A premium modern emulsion or a proper eggshell will take a wipe-down without burnishing or polishing up shiny. F&B's older formulations were never the most scrubbable — their newer Estate Emulsion is much improved, but for a busy family kitchen I'd still reach for Benjamin Moore Scuff-X or a durable Little Greene Intelligent finish.
My advice: spend the money where the eye lands and the wall gets touched. Save it on the bits nobody studies. A 2.5L tester pot of the good stuff on the actual wall, in your actual light, beats any swatch card — get one before you commit a penny.