What purpose does an antenna on a snail serve?

Normally they are called tentacles rather than antennae. They are armed with sensory receptors, including touch. In most land snails the upper pair have eyespots that can form a fuzzy image, and certainly detect movement. read more

The antenna can be pulled out to be long, but then pushed together and become short again. Things that can be made long and short like antennae are called “retractable.” The roofs of some cars are another example of “retractability” (the roof on a convertible slides out of the way). read more

Snails have no "purpose" on earth, we shouldn't have to justify the existence of something by if it does "good". Humans eat snails and snails contribute to many ecosystems. But those are just byproducts of their existence, snails did not evolve to feed humans, they evolved to survive and reproduce. read more