Because of its high defense, especially with Necrovalley on the field, Gravekeepe’s Spy will often survive being attacked face-down. This can lead to you having 2 monsters on the field to use as tribute fodder or Synchro Material.
Gravekeeper’s Spy is one of the better Gravekeeper monsters because of its ability to Special Summon from the deck which helps your swarm. With Necrovalley active your Gravekeeper’s Spy will be a decent wall with 2500 defense.
Gravekeeper’s Spy in combination with Gravekeeper’s Recruiter is good for stall while thinning your deck.