Simple answer:
As a general rule LS jerseys are generally used with arm warmers to give a little extra warmth to your arms & tie layers together better.
More complicated answer for racers & other riders were performance is important:
On colder day you have to understand most people will tend to under-dress & so are forcing their muscles in their upper body to produce more heat that they would if you were properly dressed, you don't burn more energy you just rob watts from your legs & induce higher levels of fatigue for a given effort level & duration. When in this state you don't feel cold but maximum HR is lower & you slow down more than you should do.
So when it's colder you may well need to layer on your arms. We have a problem. Layering tight arm warmers, even when fleeced doesn't improve the insulation much at all. Layering loser LS jersey's doesn't work. Combine a loser LS jersey & arm warmers then you get a significant increase in the insulation properties especially if one of those layers is windproof.
A bit warmer than that LS jerseys will tie together ever else is under the LS jersey much more effectively than arm warmers & a SS jersey. Quite often a lighter weight LS jersey can do the same job as a SS jersey & arm warmers & thus mean you don't need to be able to remove the arms because you're a weight factor down on fabric weight. This means no need to stop & the possibility of losing kit.
In warmer conditions still for all day or half day rides in the morning/afternoon you may find that a LS jersey gives too much arm insulation during the hotter periods but a SS jersey gives too little. This means SS jersey + arm warmers is the better option.
Also there are other uses for an LS jersey that a SS jersey wouldn't do. Due to the rain today I was dressed like this:
compression base layer, windproof base layer, LS jersey. Arm warmers + SS jersey wouldn't have cut the mustard as SS jersey + arm warmers would have had a separation issue & the arm warmers would have acted like one layer with the windproof base layer. A loser layer provided enough separation to allow a 'wetsuit' type effect.
i use a ss base layer and ss jersey, usually ok down to about 8 degrees
From my perspective you're probably not as 'okay' as you think you are. See above.
...certainly not Worthing wind proof no
You need windproof layers in there else you end up wearing an awful lot of kit & still feel cold.
Thermal compression layers. 
