My thought is...
The FDIC does guarantee the value of your ledger entry against bank failure to an upper limit.
If you have a pile of cash under the bed, it will steadily decrease in real value as it is ravaged by inflation. Also, tomorrow morning, the $ could be devalued and the pile though the same number and denomination of bills would buy you less.
http://www.fdic.gov/deposit/deposits/dis/
Limit is $250,000 per account - but categories are further defined. A little confusing.