And it's really useful in fantasy.
And in two ways.
The first is that the hero transitions through liminality in some way. Usually. First the character is removed from his normal condition, then he is transformed, then he settles down into a new condition. This can be your full blown character arc. It also has other transformations. Going on a journey is a liminal state, one of transition. Getting married. Having a child. Growing up -- this is one reason why the adolescent/young adult is so popular as a character. Becoming the king-- from a farmboy, of course, but also when from a Crown Prince. (Folklore has always found liminal stages in life to be dangerous, open to the peril of malefic magic.)
Then there are permanently liminal things -- places, beings, times, whatever. Crossroads, gates, ports, shores -- mountains (between the land and air). Borders between countries, localities, farms even. Dawn, sunset, midnight (sometimes even noon); New Year's Day; solstices, equinoxes and quarter days. Which can be convenient to schedule significant events -- though cliche is always a danger. (Especially for midnight!)
But liminal characters can be the most useful. Hybrids of species. The werewolf, both beast and man. The vampire, both alive and dead -- sorry, they may both be cliche, but they are both definitely liminal in nature. Centaurs, fauns, winged humans, and other part one and part the other creatures. The stereotypical blind seer or other handicapped person with powers is both less and more than ordinary humans in ability. A speaking tree is both intelligent and a plant; a talking dragon is both intelligent and a beast. Encounters with them can be numinous, transformative -- though all too often it's just part of the local color, not played for every penny it's worth.
The prize has to go to Tiraneus, when Odysseus summoned him up in the underworld. A ghost, both alive and dead; a blind seer so both seeing and not seeing; and he had been transformed to a woman and back and so both male and female. You can pile them up, but more than that you are perhaps overloading them.
What sorts of liminality have you found most impressive in fiction?