I would guess mostly double 2+ main track?
Usually it was because of an obstructing track to the right. When there was 2 main tracks would often have intermediates out to the sides rather than bridge structures, so the opposing signals would be back to back like this. Or if the line was reduced to single track, the existing masts next to the remaining track would be used.
Up here in Canada it used to be uncommon to have signals to the left of the track - rules required signals to be placed to the right of the track, and employee timetables would specifically call out any exceptions. That may be slightly relaxed now, or just documented differently (and I don't have access to modern employee timetables and track diagrams of territories...)