Two immunities, spooky movepool, genuinely creepy lore.
Ghost has two total immunities — Normal and Fighting — which is more than any other offensive type except Steel. Combined with the Ghost type's immunity to moves like Pursuit (in Gen 7+), this makes Ghost mons hard to trap and hard to revenge-kill. Ghost also has the franchise's darkest flavor text: Gengar canonically feeds on the life force of sleeping humans, and Phantump is described as a child who died in a forest.
Ghost is a fast sweeper and spinblocker type. Ghost mons block Rapid Spin (until Mortal Spin in Gen 9), so keeping one alive preserves your opponent's hazards. Will-O-Wisp burn + Hex combo (Hex doubles in power vs statused targets) is a classic Ghost support set.
Shadow Ball (80 BP, 20% SpD drop) is the default. Poltergeist (110 BP, fails if target has no item) is the physical nuke. Shadow Sneak is priority. Hex turns Ghost mons into burn spreaders with 130 BP on statused targets.
Pokémon Tower in Lavender Town is the only place in the original Red & Blue games where the background music was specifically composed to be unsettling. The 'Lavender Town Syndrome' urban legend — that the music caused children to commit suicide — is folklore, but the MIDI track is canonically a horror-game homage.