Back taxes

Back in 2005, I neglected to report additional income from freelance jobs and was audited earlier this year resulting in back taxes. I understand that fees/penalties incurred as a result of my oversight. However, I don't understand why interests is charged. From what I've researched, three different penalities are applied to late tax returns:

• Failure to file penalty - This is calculated based on the time from the deadline of your tax return (including extensions) to the date you actually filed your tax return.
  
  
• Failure to pay penalty-This is calculated based on the amount of tax you owe.
  
  
• Interests - This is calculated based on how much tax you owe.
  

I thought interest was a fee a financial institution charges to lend money. I pay interests on my car loan, my graduate school loan, my medical loan and now my mortgage. I pay late fees on a medical bill, a late fee on videos I forget to return and a late fee on picking up my little girl "late" from day care.

However, for my back taxes, I didn't borrow from the IRS. So why do I have to pay interests on my own money? I understand paying the "late" fees and penalties for not paying my taxes on time. But why pay interests when I didn't borrow from the IRS?