If it headspaces on the rim then whats the point in resizing the case in the first place?
So the case fits back into the chamber, or so it will chamber in a different firearm. Chambers have tolerances, min/max, and a sizing die is made so it will resize the case body to fit a min spec chamber. Brass fired in a "large" chamber may not fit into a min spec one without being FL resized.
Or is there no point in full length resizing for 30-30 WCF for a lever action rifle , which every reloading manual advises you do so.
There certainly is a point, its so your ammo will chamber smoothly and properly. It is recommended for lever guns, for just that reason. Generally speaking, lever actions lack the powerful camming action of a turn bolt action, With a bolt action, a slightly oversize case can be chambered and the bolt handle forced down, so the camming action locking the bolt shut forces the case all the way in. This is not possible with most lever actions, they just aren't built to allow that.
I was under the impression that full length resizing not only resizes the body of the case but the shoulder aswell.
It does. Resizing dies are made to return the entire case to min spec dimensions, (or even slightly less).
Setting the shoulder back is part of that process. However, to do this, the die body needs to be properly adjusted (per the makers instructions). Generally speaking this is with the bottom of the die body in contact with the top of the shell holder (the one it was made to use) with the press ram at full extension and all play out of the system. Not setting it up this way results in partial sizing, which MAY or may not be good enough for use in your gun(s).
You can adjust the die so its not all the way down in firm contact with the shell holder. It can be adjusted so only the neck of the case gets sized and the shoulder does not. This is called neck sizing, and some people swear by it, and others swear at it.
The common reason people do this is when they have a chamber where the shoulder is at max or even beyond, and setting the shoulder back to min spec dimensions by full length resizing results in the shoulder being worked excessively back and forth which usually results in very short case life.
The .303 British and SMLE rifles are famous for this. Again its a rimmed case, so headspace is constant and easy to maintain. However, SMLE rifle chambers are widely referred to as "generous", (to allow for function with dirty ammo & debris in combat conditions) which in effect means that the chamber shoulder is somewhere out there, far enough ahead of the spec for the case that the case will chamber easily, headspacing on the rim, but keeping the chamber shoulder cut to precise specs was not a major consideration. Remember, for military use, the case only has to go into the chamber, fire, and come out, ONCE.
Commercial rifles firing rimmed bottlenecked cases can also have this happen. Generally not to the degree of the SMLE rifles, but its always a possibility that the shoulder in your rifle's chamber could be a little "off". This has NO effect on firing new ammunition but can have an effect on reloaded cases.