As a "tire kicker" people often lose sight of the fact that a huge inconvenience can be created for the owner and their agent for the sake of being nosey.
Being preapproved does not change a buyers intentions but it does qualify them with a higher level of interest.
Before viewing homes for sale, you should get pre-approved for a mortgage. The reason is simple: How does the real estate agent (and you, for that matter), know how much home you are able to purchase? And, how do you and the real estate agent know whether or not you can even qualify for a mortgage?
If you are not pre-approved for a mortgage or if you can not show that you have sufficient liquid assets to purchase a home for cash, you can expect most good Realtors will refuse to show you homes for sale.
If you are looking at bank owned homes I suggest you be prepared to be qualified over and over by all the major lenders. Not only is it a standard to request you be pre approved by your realtor, but it is a requirement that a pre-approval letter accompany your offer and some banks(the owner of the home) will request that you cross qualify with them.
It is in your best interest to be ready with letter in hand so when you do find the home that you wish to purchase you are ready to move forward immediately.
Others have answered, but I want to ask you a question. Why would you want to see a home if you weren't pre-approved to be able to buy it? If you're just curious, and not really looking to buy please realize that home sellers have to clear out and go through lots of trouble to make their homes available to be seen by qualified buyers. Agents are trying to make a living in a tough market and need to invest their time with qualified buyers.
If you had a house on the market, filled with your valuable possessions wouldn't you want to know that you left so a pre-approved qualified buyer could see it?
There is no law, other than the law of good practice for the sake of all concerned. I hope this helps.
Pre-approval lets you, the buyer, know how much you can borrow while still remaining within a comfortable budget for your style of living.
Pre-approval lets the seller know that you can get the loan and that you are not wasting their time.
Good luck on your house hunt.