I believe that's the backstage support facility for Candlelight, which includes dressing areas and restrooms for performers, along with a full-scale copy of the stage.
Because the real stage is entirely open to guest walkways, rehearsals can't take place during park hours, and given the ever-changing lineup of choir groups in the show, overnight rehearsals aren't practical. Instead, they created a copy of the stage in the semi-permanent complex to allow the blocking to be rehearsed daily with each new batch of performers.
Like the old Hat stage in the Studios, I believe it's classified as temporary construction (since it's really only needed for about a month of the year), so parts of it have to be removed at certain intervals and reconstructed in order to meet the relevant code criteria