The Coronation is a ceremony, and it seems primarily a religious one, by which the monarch is anointed and given the regalia--the emblems of the monarchy, such as the scepter, sword and orb.  The resemblance of the orb given to King Charles recently to the Holy Hand Grenade of Antioch has already been noted by others.  It is apparently symbolic of sovereignty over the Christian world; thus the cross located on its top.  Like so much else in Christianity, though, it has its origins in pagan Rome. 
Emperors are shown holding it (minus the cross) in coins and statutes.  It denotes authority over the world, Christian or otherwise.   

Religious ceremonies may be as lavish and expensive as deemed proper by the religious, and being religious it's likely they won't be considered wasteful and unnecessary as a result.  But thanks to the cupidity and vanity of Henry VIII, the British sovereign is at one and the same time the head of the English Church and monarch of Great Britain.  The British coronation ceremony, though religious, is at the expense of the State.

That being the case, it would seem consideration of whether a coronation is wasteful or unnecessary is more appropriate than in the case of a purely religious ceremony.  This particular coronation is supposed to have cost around 125 million English pounds, if what I've read is accurate.  

I'm not British, and so am not particularly concerned by the coronation or its cost.  As spectacles go, it no doubt was impressive, from the bits and pieces I've seen.  It seems to have attracted the attention of many in and out of Britain.  Lots of people were in London for the event.  Is that enough to justify its cost?

First, does it require justification?  If our Great Nation held, at government expense, a huge Fourth of July celebration, I think there would be many here who would be in favor of it regardless of the expense.  Is the coronation something similar to such an expression of the founding of a nation, of patriotism, of pride?  

To a certain extent, perhaps, but the coronation didn't seem to be such an expression.  There may have been a time when the monarchy was considered representative of the British nation (or empire), but that's no longer the case.  It remains a source of interest, of course, but judging from what we see of it here that interest is more in the nature of gossip and mockery.  There aren't many monarchies left, and those that maintain are decidedly low-key.  Monarchs now are for the most part powerless.  That's apparently the case with the British monarchy as well, though, at least as far as political power is concerned.  But the British monarchs and their families retain a good amount of wealth, and a good amount of wealth is spent on them for their not very strenuous efforts on their nations behalf.

The coronation was an anomaly in these times.  So is the monarchy, really.  It may be seen as a part of history, and perhaps is best considered just that and no more.  It seems understandable that there are those who question whether it was (and is) worth the cost, at least to me.  I don't see how it can be considered otherwise, by any reasonable measure.  There are better ways to spend such money.  The problem of course is that it's not at all clear it would be spent usefully. 

I tend to think that money spent on the inauguration of an American president is money ill-spent as well.  That ceremony involves parades, speeches, balls, dinners to no apparent purpose as well.  It may not be as expensive as a coronation but it strikes me as no more useful or necessary than one, either.

Perhaps spectacles have some sort of inherent value to us.  We seem willing to devote large sums of money to the show itself and the excitement which surrounds it--to the experience of it.  It may not matter what the spectacle is or what it's about or why it's held.  Maybe it satisfies an urge.  This might be something else we learned from the Romans.  The state or those powerful within it would hold games and triumphs at no direct cost to the people for their entertainment, and this would suffice to make them happy and proud though they were for the most part poor and powerless.  A small price to  pay for such an outcome.