Hi there! As students we spend years learning how to obey the rules, only to see the professionals breaking the rules! You are right, the master performer is using very calculated adjustments to the angle and placement of the bow, as well as the speed and weight of the bow, and knowing what his/her violin will do. The reason he/she does this is to give shape to the phrasing and sometimes each note itself.
The short answers…, playing closer to the fingerboard gets a softer, whisper sound, usually with less weight, and often at the end of a phrase on quieter notes.
Playing closer to the bridge generally gets a more piercing, projected sound, loud, with faster bow speed and is also used to play harmonics with clarity.
Playing ‘banana’ bowing near the tip is sometimes a way to shape a baroque staccato stroke. Use sparingly!
If you have other questions about specifics, feel free to ask. I’ll try to answer.
They have trained to know how every millimeter of the bow reacts with each part of the string and they can make micro adjustments accordingly. The changes in the moment are so subtle and happen one after another so quickly that it’s not really something that can be explained, but rather “felt”.