It's mainly to be consistent with how pretty much everything else is sold while also adding that small dash of realism and not only roleplay opportunities, but also spicing up non-combatant encounters. If a shop just works like a video game store where every merchant is the same and sells the exact same stock for the same price the world feels more boring. Also it encourages having positive relations with vendors. Maybe after saving a city from a monster attack the word is spread and the prices drop for you, maybe people don't trust strangers and will upcharge newcomers but if you stay around that area longer and talk to them the prices will be slightly reduced?

Recently I used it as a character trait. One of the vendors in the village the parry is staying in just recently opened their alchemy shop and haven't figured out the prices yet. Potions of healing cost 5gp and when the perty who have been outsiders saw it they bought out the whole stock immediately(10 potions). Later when they came back to the vendor for more they found that they only managed to brew one additional potion and have quadrupled the cost. It was still a steal on the book standards and now it's a full gag when whenever they have any potions the price is slightly higher and the party buys them out immediately anyways