Making a big investment like buying a home is stressfull enough, right Maureen? So it's a good idea to get your finanical 'ducks in a row' before you start searching. You might fall in love with a $200k home only to find out there is no way you can afford one over $150k. Additionally, you might find out you are approved for $250k but once you hear that number and go over your typical expenses (including those vacations you like to take), you might only be comfortable in the $175k range.
Secondly - sellers and listing agents for seller's expect a qualified pre approval letter to accompany a purchase offer. So you may as well be prepared in case you see a home and want to write an offer on it that night.