There are two main reasons why a generator needs to be polarized.
1. It has sat for an extended time and the field coil iron cores have lost their residual magnetism.
2. While working with the charging system something may have accidentally reversed the polarity of the system.
The positive or negative marking on the regulator has nothing to do with controlling or maintaining the polarity. It indicates that the regulator is designed to work at that polarity, which is often due to type of material in the internal contact points.
Polarizing instructions often show doing it at the regulator. This uses two easily determined contact points and it keeps the contact spark away from the threads on the screw on the armature post on the generator thus preventing arcing at the screw and damage to the screw threads. Polarizing can be done from any point that makes connection to hot battery post to any point that makes contact with generator armature post.
Note that this procedure is for Type A systems. Type B is done differently but that is another discussion.