Gas Bottle Regulators what are they for?

Yep, that reply is spot on ianmc.

Also, the Coleman having no restrictor jet results in a much better control over flame height from full to simmer.

I ditched my Companion stove (same principal to Primus) on my return from Corner Country where bulldust had completely blocked two out of three jets. Bought the Coleman and it has been worth the investment.

or even Boyle?

Him too... we shouldn't forget Lance.

The pressure in the cylinders will remain fairly constant whilst the LPG is still in liquid form in the cylinder. Usually around 150KPa, some appliances such as the screw on burners can cope with this by having very small injectors (which can get blocked very easily). Most domestic LPG appliances run at 2.75KPa and therefore need the regulator, theses would include a lot of BBQ, camping stoves etc which get fitted into CTs and caravans. Using an appliance that needed a regulator without one would be very dangerous as the fittings, taps etc are not rated to take that pressure and the flame characteristics would be all wrong.