The basics are the same as any other cartel. OPEC states cover enough of the supply-side of the market to be able to keep prices artificially high.
UAE leaving means UAE can price below OPEC's target and take more of the market. OPEC will have to react and lower prices or concede some of the market.
Does any of this matter if the major players can't ship oil through Hormuz? Who knows...
OPEC was never a very effective cartel in the first place. Many of the members routinely exceeded production targets. And for geological reasons it's not like most oil wells can even be throttled down.
Storage facilities allow for market supply control.
And while it's true many member exceed targets, it's like speeding on US highways: everyone does it, but anyone driving 20 mph faster than the pack is nobody's friend. Karma will happen.
None of the OPEC+ members have sufficient storage facilities to allow for meaningful market supply control. And karma isn't a real thing: most OPEC+ members are more like rivals then real allies. They'll agree to one thing and then do another, secure in the knowledge that there will be no serious consequences.
The UAE is trying to expand its ability to ship oil through Fujairah, so this could potentially undermine both KSA/Iraq and Iran.