You owe taxes all year round. What you're doing in April is settling up and filing for the year. If you run your own business, you're required to pay a lump estimated taxes 3 times a year in addition to your annual filings. If you're too short, you owe and you owe interest. The IRS has a safe haven rule where if you pay 90% of what you owed this year, or at least 100% of what you owed last year, they won't penalize you. It's actually one of the reasons I personally do over withhold. I do some contract work on the side, and rather than calculating and sending in estimated taxes every quarter, I just have my regular job send in about 25% of the contracting income I expect to make. On years when I did as much contract work as I expected, I basically get nothing back or I might owe $200. On years where I don't, sure I gave the government an interest free loan but I also didn't have to think about my taxes for the whole year.

If you get a refund it means you overpaid your taxes. The amount you've overpaid can be considered a zero interest loan to the government. If you hadn't overpaid your taxes you could have invested that money.