As a young child I always thought they did this to wipe the sweat off their brow and get a drink of water. Later in life, speaking now as a solo performer, my own reasons for this are to regain my composure, wipe the sweat off my brow and get a drink of water :grin:!
A performer will always give 1000% for a solo performance, utilizing every ounce of energy possible while playing. Once a major piece has completed, and the thunderous applause begins, it becomes a very emotional experience ... at least for me ... and I need that little "time out" to gather my wits.
It will be interesting to get other peoples views on this topic.
Yes, they do need that little "time out" to wipe the sweat off, take a breath, relax a moment, give each other a pat on the back or say a couple of words, then go back out to acknowledge further applause (which doesn't always happen). Similar to theater when the curtain comes down and then comes up again, and sometimes down and up again.
Also, if it's just the conductor and orchestra, the conductor will walk off after a piece is done partially as a symbolic "slate cleaning" (IMO) and to mentally prepare for the next piece, but also because often times the orchestra needs to either add or subtract players depending on the needs of the piece.