Very little of importance happens during or is achieved out of troop movements during the opening.
In that regard, a bounce can be a highly-satisfactory outcome between two neighbors who are not intending to begin a first season major offensive against each other.
I mean, I pretty much need to capture one supply center the first year, but that's it. My only other real objective during that year is to find out who the idiots are. One way that idiots can be easily identified is that they will frequently lie to you during the first season.
By offering a bounce to one of your neighbors, you signal to him that you are not an idiot, and that you think he is not an idiot. This becomes important because the first objective of senior players is to eliminate idiots from play except where they appear rather easily manipulable.
The other nice thing about a bounce is that it usually prevents you from becoming squashed during a surprise attack.
Remember bounces are usually a temporary situation. If you and I bounce each other, meanwhile a third party may attack you and a fourth party may attack me. It's then logical for me to assume that you're going to leave me alone to deal with the person attacking you, and my optimum response will be to leave you alone to defend myself from the guy who's attacking me.
Obviously, idiots can neither follow nor employ that sort of logic. Some of the most difficult (and rarest) skills for an operative to develop in response to a threat-heavy environment (e.g. the Diplomacy board) are risk assessment and risk management.
But by offering (or accepting) a bounce, you demonstrate to your neighbors that you possess basic credibility and that you're not a total douchebag. That's what people worry about - you'd be surprised how many noobs lack the wherewithal to follow through on a simple commitment, even one that will cost them nothing. By offering, accepting and then honoring a simple agreement, you stand out from the crowd.