Why do I play Qe2 in the Potter Variation of the Scotch Game?

In the Potter Variation of the Scotch game, I have noticed that the recurring theme of playing Qe2 blocks in the bishop, and yet is still the best move. What is the point of blocking in the light squared bishop and how am I supposed to develop the bishop in the future? Also, please give me some key ideas about why Qe2 is played in general right off the bat. Thanks.

Here is what I am talking about: 1. e4 e5 2. Nf3 Nc6 3. d4 exd4 4. Nxd4 Bc5
5. Nb3 Bb6 6. Nc3 Nf6 7. Bg5 O-O 8. Qe2

8 Qe2 is not the only move: 8 Bd3, 8 Bb5, 8 Qf3 also come into consideration
The idea of Qe2 is to prepare O-O-O

Couldn't I just play Bg5 first and then Qd2; or even Qf3? That way I can queenside castle and easily develop my light squared bishop.

7 Bg5 O-O 8 Qd2 h6 white must capture 9 Bxf6 losing the bishop's pair
7 Bg5 O-O 8 Qf3 has been played, black goes 8...Nd4 or 8...Ne5 attacking the queen

For some reasons I like to play 7.Qe2! which is even stronger than 8.Qe2. I learned those lines from the course.

Sorry Susik, what course are you refering to?