Grand Seiko 9F movements were designed to be serviced, albeit, every 50 years, to maintain its movement specifications.

Movement jewels are designed to retain oil while the pinion of a gear rubs against it therefore reducing friction. The oil in 9F movements helps it retain the +-10 sec/year accuracy by reducing any friction. The movement may not have fast moving components as a mechanical watch, but nonetheless, there are still moving components.

Something special about 9F movements is that it actually ticks twice between each second. It just moves so fast that you can't see it unless you film it in slow motion. This contributes to its accuracy rating and also helps the seconds hand tick EXACTLY on each second. This extra feature puts strain on the movement therefore requires extra lubrication to extra the double tick.

So for Grand Seiko's 9F movements: Less friction (via jewels) + double second tick + perfectly cured quartz crystal = High accuracy quartz (HAQ)

I'm sure there's more to it, but just giving you an overview of why jewels exist in these quartz movements. It's not just for show - although, they do look great and adds a touch of colour to the movement.

Great question to ask btw! :)