There's 3 options Eva has with the SWP, ranked most to least likely:
1. It doesn't get played at all. All the votes are too important.
2. She uses it on herself (or Joe, they're a duo so it's one and the same). If she and Joe get spooked at all that Shauhin or Kyle aren't 100% into alliance, the one that doesn't win immunity could use the SWP and keep the idol for later.
3. Use it on someone else (not Joe). Unlikely option, but this could be the move that wins the game for Eva. She has a great story at final tribal, but imo she loses to Joe right now. She needs a move to separate herself from everyone in the game. She held the idol basically the entire game. Then she gets this SWP, give it to Mary at 7, make a secret side alliance with her. Use that to eliminate Mitch and Kamilla in next 2 votes.