Bail arm and line roller
On a spinning reel the bail does the opposite job to the line going out. On the cast it is folded away and takes no part in anything; on the retrieve it catches the line coming back and lays it onto the fixed spool.
Two roles, not one
That is why the assembly looks complicated for so simple a task. The bail has to get out of the way completely while line is flying off in coils, and come back just as completely when it has to be picked up again.
That is also where line twist comes from: every turn of the bail adds one turn to the line about its own axis. What to do about it while filling the spool is covered separately — how to spool a reel.
Why it is closed by hand
The bail closes in one of two ways: by itself, on the first turn of the handle, or by hand. By hand is better, for two reasons. The first is the impact: an automatic close throws the bail over with a spring, and that blow lands on the bail itself on every cast.
The second is more serious. On an automatic close the line is picked up by whatever happens to catch it, and can end up lying past the roller — straight on the bail wire. You will not notice at the time, but you will on the first take.
The spring: what happens to it
The bail spring weakens with use, and it weakens gradually. First the bail stops clicking decisively, then it fails to close fully, and after that it begins to close by itself during the cast — the most expensive failure a reel has, because the lure goes with it.
There is one check: the bail must snap into the open position with a clear click and stay there when the reel is shaken.
The roller is checked with a finger
The roller should turn freely at a touch. It seizes not from wear but from dirt: sand collects under it, sometimes line or hair winds around it, and the assembly stops turning altogether. It is worth a look every time the tackle is put away — reel care and storage.
The oscillation gear and ridges on the spool
While the rotor turns, the spool travels back and forth — a separate oscillation mechanism does that. If line builds into a ridge near the front lip or near the base, the spool position is corrected with washers placed beneath it.
Uneven winding is not cosmetic: a ridge throws a loop on the cast at precisely the moment the line is moving fastest.
Why the other types have no such assembly
On a baitcasting reel and on a centrepin the spool turns by itself, so there is nothing to catch the line with — it winds on directly. That is why there is no bail, no roller and no line twist there, and why instead there is a backlash, which a spinning reel never has.