Trance-like states? Music, drumming, and ecstatic dancing? Intoxication by wine?
Sounds like a party!
Perhaps in many ways it actually was. That said, the Baccanalia was at the same time quite a meaningful birth-death-rebirth religious celebration that came to Ancient Rome via Greek settlers from the south and Etruscan settlers to the north, who brought with them their fertility cult rooted in the Dionysian “mysteries”.
Why mysteries? Well, the Baccanti, the women worshippers of Bacchus, would secretly get together for “enthusiasm”, from the Greek en-theos-iasmos, i.e. possession by god. The general idea? In a nutshell, better to acknowledge the dark and mysterious bits that exist within us (as opposed to rejecting them) if humans are to ensure their psychic well-being.
Pretty serious stuff, perhaps not just a party after all.
Of course, had the “mysteries” merely been a silly party, the authorities might have left everyone alone. But, given the cult’s secret nature, the powers that be eventually came to see the celebration as dangerously liberative and far too egalitarian. (Roman citizens interacting with non-Roman, all ages, run by women, etc.) So in 186 BC, the Senate puts the kibosh on it all by enacting the Bacchanalia Reform, removing all the meaningful aspects and leaving the celebration a shell of its former self.
Yet some things are too elemental to disappear altogether, and sure enough the Baccanti live on represented and referred to repeatedly by philosophers, poets, and artists in study, song, and art throughout the ages.