Frogs do for the night what birds do for day — make our natural world musical. The vast majority of the frog calls we hear at night are 'advertisement calls' or vocalizations from male frogs 'wooing' female frogs. The female selects the call she like the most and will chose that crooner to mate with. Female frogs can vocalize, but most do not call to attract a mate. Furthermore, the female frog ear is finely tuned to the call of her species, so what might sound like a jumble of multiple species calls to us, is not confused in the signal to the female frogs.
In some species, there are also sneaky 'satellite males' which perch silently next to singing males in hopes of swooping in and stealing a mate attracted by the calling male — dirty.
For the purposes of amphibian monitoring, frog calls are extremely useful. We can record an empirical answer if an area is occupied by a species by their call, as well get an estimate of abundance. MAAMP surveys always begin with a 5 minute silent auditory survey in an attempt to detect and identify frogs by their call.
Below are all the frog calls from the Atlanta region of Georgia. These calls have been painstakingly recorded by Walter Knapp while he was conducting surveys for the GA Herp Atlas, and used by permission for this research.