Why is the bride (& groom) held up on chairs & danced around after the ceremony?
This is done for the purpose of bringing the bride and groom joy. In addition, a groom on his wedding day is considered as a king, and the kalah his queen. This status is perhaps alluded to by the “royal entry” on their respective thrones.
I do not know of any halachic (Jewish Law) reasons for the practice.