If Charles is already declared the king why is a coronation necessary? Or I should say why is a big event necessary?
It isn't as seen with the fact that neither Edward V nor Edward VIII ever had a coronation but were still the monarchs.
It was necessary in the early middle ages as it was really only when crowned that the person became the monarch which is why the coronations were often held so quickly but now it is largely symbolic.
It is a tradition ... one that goes back well over a 1000 years and always has been a big issue. The homage was very important as it tied the peer to the monarch in a very public way but the last full homage was at Queen Victoria's coronation.
The UK also knows that a big coronation will bring in a lot of tourists and thus a lot of pounds as well as showcase the UK at its best.
I have heard a number of Aussies say they want to visit the Scottish highlands now after seeing the drive from Balmoral to Edinburgh. They didn't realise how beautiful is it. Others have said they would like to go to Cardiff and Belfast due to the images from Charles driving through those cities.