If you are recording video and audio in situations where the mics must be invisible, and cannot be placed close to the subjects, a boundary mic (or two for stereo) on the stage floor can come in handy.
Some examples are theater productions such as plays, musicals, and opera (although you might ask for a feed from the house mixing console — use a transformer isolation box in that case). Other examples are concerts or recitals of small musical ensembles such as soloists, duos, quartets, etc.
If you record a wedding in which lavalier mics are not acceptable, a boundary mic on the floor near the participants is a useful option.
Bruce Bartlett
https://www.bartlettaudio.com