Garth Allen, "Your Powwow Corner,"
American Astrology September 1957:
...the supreme fact of astrology is that the mundane structure of any horoscope has a modifying effect on the intensity of all planetary forces... It is only in the zones centering around the three basic great circles (horizon, meridian and prime vertical) that the inhibiting pressures are relieved or removed
I wanted to post this quote because it doesn't get any simpler or definitive than that! He elaborated (with respect to transits in particular):
A promised or indicated event can more easily come to pass when the aspect is quite platic [non-partile] but in a strong position in the horoscope than when it is exactly partile and weakly situated. It is when the two conditions, closeness and angularity, occur simultaneously that a crisis is virtually inevitable.
This very accurately states what we've observed: That transits tend to peak when partile (and especially when exact) but, even when non-partile, they become a really big deal when they fall on angles of (say) the current lunar return. Furthermore, when exactness of aspect and close angularity combine, you get the outstanding emergence of events that he called "virtually inevitable."