Money's value is as an exchange medium.
The more concentrated the money becomes, the less it is being exchanged.
When less money is being exchanged, the economy grows more stagnant.
If the economy continues to grow more stagnant, recessions typically take hold -- and deflationary pressures begin to realize the underlying devaluation of the currency and reflect that by lowering real purchasing power.
Suppose one person manages to hoard all the money (essentially this is the condition of Gini Coefficient = 1). Then, money is worthless. It ceases to have any exchange value.
Sadly this thought experiment isn't as extremist or far-fetched as one might think, considering the stats:
The three wealthiest individuals in America own more than 50% of the domestic populationThe eight wealthiest individuals in the world own as much as half the entire planetAnd this doesn't even begin to broach the topic of how overly concentrated money affects political power...