The Lions run a 3-4, so they needed different defensive linemen to begin with. They needed a nose tackle, and got the only one in the draft. That's a lot of position value on the 2nd day.
Just a reminder that the tank is going well, they should have the #1 pick sewed up soon. They have room for error, so they can win a game, preferably against Chicago, but that's not entirely necessary.
Semien isn't really a SS right now. I know that Avila's stated measure for the position involves making all the throws. I think he would lean toward SS's with really good arms. Correa and Story fit that bill, and Baez a bit.