I don't understand how the Support boxes are filled with support. I see in the beginning that certain Support boxes have counters placed in them, but how and when are the Support boxes refilled? For example, the Spanish Treasury begins empty as do one or two others. How do they get filled? Have read the rules numerous times and I don't see anything about this. What am I missing?
Units in support boxes will get you more units to place the next turn. Some support boxes allow you to pull units out as money instead of putting them on the board (in addition to the beachhead idea noted by Jan).
For the Reformers in particular, at least in the games I've played, units in support boxes are a serious threat. Reformers can use units in those boxes to sprout up all over the place.
The board specifies which factions are eligible to place units in each support box. Color-coded as well as written on the board itself, I believe.