Well, Lurker does two things.
The first (and more important) is gaining action cards from the trash. Two of them is like a super-workshop that can gain good actions like King's Court and Grand Market.
The second is trashing cards from supply. This gives Lurker decks a lot of pressure on the piles and leads to shorter games that involve dancing around lowered piles.
Trashing cards from supply also leads to combos with cards with on-trash abilities like Fortress, Catacombs, Hunting Grounds, and Cultist. These combos are quite powerful and generally worth prioritizing early on.
Without a combo, Lurker is a fine-but-middling card that acts like half of a super workshop. In engine decks, you usually want to trash first, then add Lurkers as payload. Somewhere around the mid-game, you'll have to make a judgement call on whether the game is likely to be determined by piles or total VP. If it's piles, add additional Lurkers. If it's VP, stay at just a few Lurkers and build a deck that has enough money to buy VP.
A lot of it depends on the board, but a lot of playing Lurker well comes down to paying a lot of attention to the threat of three piling endings.