Confused about the generator design?

[gzrjzcx]

Hi,
Im new here and I am a little confused about the tick() generator design after skimming through the code roughly. I cannot figure out the case which I must use the iterator to visit the whole tree? I have found the iterator symbol in the whole repo, while I cant find the use of this symbol to do something. Would u mind explaining the idea of generator for me?

By the way, is it tick a node and return its result status would be a more intuitive way?

Thanks