After the opening moves
[fen ""] 1.e4 e5 2.Bc4 Nf6 3.d3 c6 4.Nf3 d5 5.Bb3
one of Black's fashionable answers is
What is the aim of this seemingly useless check?
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5...Bb4+ looks like a weird move because you wouldn't play a move like that often, but it makes perfect sense because the white bishop in on b3.
White would definitely want to develop his knight with Nc3 on the next move and put pressure on Black's pawn center. By playing 5...Bb4+ you prevent 6.Nc3 (it'd lose a piece). If White goes for 6.c3, then Black can put his bishop on d6 without his position in the center being at jeopardy.
That's why the most successful move here is 6.Bd2!. Now 6...Bd6 is met with 7.Nc3!, so Black usually prefers to trade bishops.