Oh My God! I completely understand now. When Cas had his grace taken from him he was then seen to be wearing a hoody and jeans and wandering the streets without a purpose. Now he’s got his grace back he is now in the suit and trenchcoat…meaning the old Cas is back, the Cas that we first met and came to love. This is symbolism at its finest.
But seriously, you dont realize how happy I am that the trenchcoat is back.