A great deal here depends on the developer you're using. In this case, there are three key ingredients. First, is the fact that it's Rodinal. Rodinal is a one-use developer, meaning that you use it once, and then throw it away, because the chemicals get "used up' in the course of developing one batch of film. Second is the high dilution, to keep the amount of chemical available in any one part of the film to a minimum. Third is the minimal agitation -- basically, just enough to give a reasonable assurance against air bubble forming on the film.

With a typical developer, you control the amount of development by the time you let the film sit in the developer. The developer is concentrated enough that the longer you let it sit, the more it develops.

With Rodinal at 1:100, you're basically just letting it develop until the developer is used up. If you were to let the film sit in the developer for another hour or two, it wouldn't make much difference either -- the developer is pretty well used up by then, so if you let it sit for (say) another hour or two, it wouldn't change much more. The lack of agitation means that (for the most part) as the developer gets used up, it stays close to the area of the film that used it up. In areas that had different exposure the developer will be used up at a different rate, so you (mostly) want to keep it in place, to assure against less-used developer getting redistributed around to places that it would continue development too long.

A different developer (e.g., D-76) would change the situation entirely though -- most other developers contain enough chemical to develop quite a bit of film. Even if you minimize agitation, if you left film in D-76 for an hour, it would be so over-developed it probably wouldn't be usable.

Summary: in the case of Rodinal, the development time isn't critical because the developer itself is basically self-limiting. With most other developers, however, the amount of development is controlled by the time the film is in the developer, so the time and temperature are critical to control the total development of the film.