One of the most difficult things about travel is managing your money. Paying for meals or souvenirs in local currency can lead to your paying too much for your purchases.
Currencies fluctuate in value every day. Also, each currency is worth different amounts. With the variations in values and rates, it’s easy to lose sight of how much a Euro (or Pound or Yen or whatever) is worth. If you forget to convert an amount of local currency into your own currency, you can drastically overpay for something.
I personally use a site called XE (www.xe.com) to help me keep track of my currency exchange questions. The site allows you to convert between virtually any two currencies. Although it may be a “one trick pony”, the site can give you peace of mind in your travels by keeping you aware of the true cost of your purchase.