The tongue was a body part used for tasting and communication.
After finding and taking a black orchid while on an expedition in the Amazon, botanist George Cranleigh was captured by the Kojabe tribe. Believing the orchid sacred, they mutilated Cranleigh and cut out his tongue for the crime of theft. (TV: Black Orchid)
Some Time Lords could determine what was in a substance by touching it to their tongue. Romana tasted a mineral with her tongue on Skaro to guess its composition. (TV: Destiny of the Daleks) The Tenth Doctor was able to tell not only that the Sycorax were using blood control, but what blood type they were using by tasting it. (TV: The Christmas Invasion) While visiting Queen Victoria, he could tell there was mistletoe oil on some woodwork by licking it. (TV: Tooth and Claw)
Certain species could use their tongues as weapons. Tetraps had a forked tongue which could inject a paralysing poison. (PROSE: Time and the Rani) One tribe of Silurians encountered by the Eleventh Doctor lacked a third eye, as did others of their species he had met previously. They did, though, have long tongues which they could flick to inject a venom, which was mutagenic to humans. (TV: The Hungry Earth/Cold Blood) Zygons had venom sacs in their tongues. (TV: The Day of the Doctor)