A pre approved mortgage allows you to know how much of a mortgage you qualify for and how much of a house you can afford. When you pre-qualify for a mortgage, you get a guaranteed rate for 120 days. If interest rates rise, you have a locked in rate. If interest rates drop, you will automatically get the lower rate.
Getting pre-qualified for a home loan will give you an idea of what purchase price you want to spend to get payments you are comfortable with. A pre-approved mortgage makes the seller feel more confident by knowing that you are a serious buyer and gives you more leverage when negotiating a purchase price for your new home.