Its a plant. Guppy grass is a pretty easy plant to grow, as such its pretty common the hobby because its not demanding.
Plants in an aquarium are desirable because
Plants use nitrate and phosphate (from fish waste) and to a lesser extent calcium and magnesium (reduce water changes and improve water quality).
Plants expel oxygen through their leaves as a result of photosynthesis, increasing the amount of O2 available for your fish and making a GREAT surface for the colonization of nitrifying bacteria (which helps augment your bio filter)
They provide a hiding spot for fry so they don't get eaten.
And they look pretty.
What's not to like?
If you have enough light on your tank to grow guppy grass (or other live plants) I highly recommend them.