Switchable-pattern large-diaphragm microphones are always built with dual-diaphragm capsules. In the "cardioid" setting, such capsules have a pickup pattern that's cardioid only in the midrange. The pattern always broadens out at low frequencies (more so in some designs than others), while becoming narrow at high frequencies. See for example the attached set of polar diagrams for a much-sought-after vintage studio "cardioid"--the broad pickup pattern at low and low-mid frequencies is part of what gives "warmth" when recording vocals in a studio.
If you set up a closely-spaced or coincident pair of such microphones for stereo, though, the cardioid setting is a very poor choice giving both uneven localization (due to the "beamy" high frequencies) and a lack of spaciousness (due to low-frequency response that's nearly mono; broader pattern = increase in the same information being picked up in both channels "in phase").
Cardioid is the "default choice" that most people fall back on--though you might chalk that up to inertia and conformism as much as anything else, particularly with microphones that have four or more patterns available (a very big advantage over the usual three). Supercardioid and (for non-coincident setups) "wide cardioid" can be really excellent choices for two-microphone stereo recording, and if you look at directional patterns as graphed against frequency, you will often find those patterns to be smoother and more consistent.
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