Why is it significant that much of the nation's wealth was owned by a small number of people?

Which nation? I take it you mean the United States or Great Britain?
Because a few people, unless they invest their money into capital, can only buy so much. How do you justify the purchase of multi million dollar race horses. Who benefits from such a purchase?

How do you justify the purchase of a mansion with 12 bathrooms? Who needs such a place except a family of 13 children and 2 parents?

However, if they are conscientious and spend on capital and business interests, they put their wealth to work and and create jobs which helps many people.