Sometimes in the Bay Area there is a marine layer that extends below 3500 feet. Thus, the pilots can't see each other by the time they arrive at the 3500 feet limit and SFIA can only land one stream of planes. SFIA is built at the worst possible place in the bay area, right at the Bay end of the San Bruno gap; that makes the weather at SFIA closely connected to the weather at the ocean at the other end of the gap.

SOIA, which uses PRM, is a landing technique that allows the planes to descend to a lower alitude before the pilots are required to see one another. It's allowed because, with PRM, the Air Traffic Controller can be more certain that the planes are far enough apart to avoid one another. The ATC folks talk about a No Transgression Zone(NTZ) between the planes. See below for a diagram.

This graphic is from a presentation by the Technology Panel. AGL is Above Ground Level, MAP is Missed Approach Point, and SAP is Standard Approach Point. LDA is Localizer Type Directional Aid.

So just how often could they use this?