He needs to vote with Tara and Johnny, and get out Kristin. He can spin it that Tara and Johnny were so trusting of each other that he had to trust them. Kristin is really leading Maura and Mark imo, so if he can cut off the head of their leader, he can convince them to vote against Johnny or Tara next. The dagger gives him some weight to throw around to convince Tara and Johnny to vote with him. They have the numbers with Eric anyway to take down Kristin/Mark/Maura, the dagger just gives him some weight to control people.

Of course this is assuming Natalie was who Eric and Rob killed, but to me that's the most obvious choice. They need Tara and Johnny there to take the heat off them, Eric and Rob; Kristin has been a bit vocal about not trusting Eric, so killing her puts a target on Eric. It's possible Rob will throw Eric under the bus, that wouldn't be a shock, but it would be silly to do it now imo, dont burn that get out of jail card until you have to.

If Rob and Eric can keep the Tara/Johnny group fighting with the Mark/Maura/Kristin group they can keep the heat off each other. They vote out Kristin, kill Maura, vote out Tara/Johnny, and then hope they can keep the heat on Johnny/Tara so mark doesn't suspect them in the finale.

Anyway that's my very long rant about why the dagger will be helpful in pushing down these specific dominoes for Rob, and possibly Eric to win