The Gemara Megillah 12a says that Achashverosh wore the bigdei kohen gadol.
I dont understand why rashi uses the terminology that he uses
בגדי כהונה - שהיו בידו שם בגדי כהן גדול שהביאן מירושלים: "The priestly vestments: Since he had in his hand there, the vestments of the high priest that they brought from Jerusalem."
What is rashi adding or explaining that isnt known from the pshat? Isnt it obvious if he was wearing the levushim, that they were from yerushalim?