I suspect there are two reasons for this. One is Bernards need to be something and to impress Lenina. The trip gets him some time alone with her, a chance to impress her. But I don't think that's the most important thing. The Savage and all he stands for needs to be brought into the story; for that to happen, we have to get to the reservation. Sending Bernard there solves two purposes --- it adds to the sense of Bernard's frailty and need, and it advances the plot.
When Bernard gets back, he is a hero for a while ... he gets some of the attention that he craves, but this is just a setup. It helps make him stand out, and his fate reminds us what happens when "the individual feels."
Bernard goes to the reservation for "a vacation". Because Bernard is so different from other in the Brave New World, he is attracted to the idea that the reservation will also be different. He has just be scolded by the Director for his lack of "infantile decorum". In essence, Bernard is criticized for being too mature. The reservation is especially attractive to Bernard because the "decorum" will be different than that found in the Brave New World.