Exactly. They knew FISA judge would never approve tap on Trump, but they hoped to catch Trump or Team Trump in something when they listened to whom Page was talking to.
At this point, now that I’ve read your comment, it makes me wonder if the judge would not have approved a tap on Trump because there was no reason, or because his hands would be dirty and he wouldn’t have any cover. It is entirely possible that the judge only required something to give himself “plausible deniability”. I.e., so he can point to the application and say it looked legit to him, that he was a victim of the lying application, not complicit in the wiretapping.
It would be interesting to know the real story there.