I have already read the section here on the Evans Gambit but I still don't understand the value of sacrificing the b pawn unnecessarily, in the likely case that black's Bishop returns to the c5. White could have simply proceeded with c3, d4, gaining center control and attacking B. Why interpose losing a pawn? If it is to open the c1-a3 axis for B, then b3 should be played. Can someone explain why giving away the pawn, while black can simply take it and resume where it was, is a good move? Unless there is a need at that time that can be filled by giving away a pawn, I fail to see the value of this move.
Compare the position after
3...Bc5 to that after
4.b4 Bxb4 5.c3 Bc5 6.d4.
[fen ""] 1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 Bxb4 5.c3 Bc5 6.d4`
It's true that the Black bishop has simply returned to c5 and can retreat safely again, but note that that is all Black has gotten to do while White got in the central c3/d4 push. In simplest terms, the point of the Evans Gambit is that gain of time, getting in those moves to expand in the center (and open lines for potential development/attack) without Black continuing her development in the meantime. Such things usually aren't free of course, and here it has cost White the b-pawn. White has traded in a bit of material in attempt to have greater dynamic potential and the initiative.
White gains time to gain control of the centre after Bxb4 by c3 which can be followed by d4 at some point. Black's bishop will be kicked back from c5 then White can develop an initiative. Also after c3, Qd1-b3 is a possibility attacking both b7 and f7. This gain of time for development can put Black in an uncomfortable defensive position.
The purpose of any gambit is to sacrifice material for time. Since time is an ephemeral advantage, naturally you must capitalize on it quickly or you will most likely lose in the long run due to your material disadvantage. Methods of neutralizing all the gambits have been discovered, generally by returning the pawn at the earliest opportunity, but in the heat of battle it isn't always possible for your opponent to find the correct move, and it's worth trying them for that reason, particularly if you are strong at tactics. If you like to have the initiative and attack, playing a gambit isn't the worst thing you can do. Of all the gambits, the Evans has proven particularly potent, i.e. when you are given the opportunity to play it. Some of the most brilliant games on record are Evan's Gambits, such as Anderssen's "Evergreen Game". The previous answer here addresses your specific question about the Evan's.