Why did the women in Sura 12.30-31 cut their hands when they saw how "beautiful" Joseph was?

I'm reading Sura 12, the narrative on Joseph's life, and I don't think I quite understand the **historical context for why the women at the banquet cut their hands with a knife when they saw how handsome Joseph was?** Was that a cultural custom/expression? **I also don't understand why Potiphar's wife held the banquet in the first place, and what the function/purpose of it was?** Can someone help clarify this scene for me? Thanks!