This card is not just used in Gravekeeper's decks. In fact, it is used more so outside of Gravekeeper's decks. Most people will put two "Gravekeeper's Spy" into their deck simply to search one out with the other, creating a wall of 1 or 2 2000 DEF monsters.
This card is commonly used to easily generate tribute fodder for "Monarchs". Flip one, use its effect to search out another (or sometimes "Gravekeeper's Guard", with a respectable 1900 DEF), and then tribute the one you flipped for a "Monarch". Notably this can still be done fairly safely if your opponent has a clear field; you can remove one of them for the effect of "Caius" to do some burn damage to the opponent (since all of the "Gravekeeper" monsters with under 1500 ATK are DARK), or put a Spy on top of your deck with "Raiza the Storm Monarch" (which, if you are running more Gravekeeper's than the standard 2 "Spies", will allow you to Set a "Spy" next turn to flip over later for yet another "Gravekeeper's").
One good combo for this card in a Gravekeeper's deck is that when you have "Gravekeeper's Cannonholder" on the field with "Gravekeeper's Spy" you can flip "Gravekeeper's Spy" face-up and summon a "Gravekeeper's Curse" to do 500 damage and then use "Gravekeeper's Cannonholder" to sacrifice "Gravekeeper's Curse" and do 700. This can repeated if you have something to flip "Gravekeeper's Spy" face-down again (such as "Tsukuyomi" or if you have another "Gravekeeper's Spy") and it can do up to 3600 damage, you can also launch your extra "Gravekeeper's Spy" to do 700 damage each.
You can improve this card's speed by using "Book of Taiyou", to activate this card's Flip Effect on the same turn you Set it.
This card's effect can be used to Xyz Summon a Rank 4 monster somewhat efficiently.
This card can special summon a gravekeeper's monster with 1500 attack or less in any position, using allowing swarming of the field with multiple gravekeeper's spys'.
Consider using "Acid Trap Hole" on this card while utilizing "Necrovalley". It will activate the FLIP effect of this card while also returning it face-down to be able to use its effect again. This can become an easy Rank 4 with 3 Materials (use it during your opponent's End Phase) or a strong defense as noted above.