The GE was originally introduced as a key aspect of restricted trade - the game system disallowing unbalanced exchanges of wealth, either through trade or PvP to combat goldfarming - along with the removal of the real wilderness. Trade value was calculated from GE value, so a trade would only go through if the GE value of the items on both side ~matched.

The +/- 5% thing was at least originally partially intended to prevent players pumping obscure items on the GE via wash trading, then using that inflated GE value to transfer gold through trades that look fair to the system but are unbalanced in actual economic reality.

That sort of unbalanced trade still ended up happening via "junk trading" - players would find items with fixed, high GE values (e.g. addy arrows p(++)) but low actual value, and conversely items with low, fixed GE values (e.g. mint cakes) but high actual values, and use those to circumvent the balanced trade systemm.

In OSRS, on the other hand, adding the GE was fairly uncontroversial because it wasn't introduced along with restricted trade, and because it replaced third-party marketplaces like the Zybez exchange that were shitshows most people disliked having to use. That version of the GE functions as you describe.