What is the main purpose of the Evans gambit? When I see the Evans Gambit, I don't see the point of it, maybe because I am a beginning player.  It seems that after:

[FEN ""]

1. e4  e5
2. Nf3 Nc6
3. Bc4 Bc5
4. b4  Bxb4
5. c3  Bc5


Since the bishop can go back to c5, it seems we are just back to the beginning and white has already lost a pawn.  After watching some videos on this, I have seen that white can build a strong attack if the queen moves to b3 which can then backup the bishop on c4 and eye the a1-h8 diagonal, and the critical f7 square.  Is this it's main purpose (especially if it is accepted)?  If it is, why is it accepted more than declined?
Black's choice of where to put the Bishop on move 5 is awkward, and any choice will give White some compensation:


5... Ba5 would give White the possibility of Ba3 at some point to cut off the King from castling and the Bishop won't be very useful after White castles out of the pin on c3.
5... Bc5 allows 6. d4, taking control of the center with tempo after which White can develop quickly and try to attack.
5... Be7 is rather passive and blocks a square for the g8-Knight, and White can develop very naturally and aggressively.


The reason White is better off than before move 4 is that the c3-pawn supports d4. d4 is very strong especially when White can gain a tempo on the Bishop.

Declining it with 4... Bb6 is not unheard of, but 4... Bxb4 is generally considered more challenging.