Almost all offers have to have either a letter of pre-approval for the loan or a statement from a bank showing the cash amount to purchase is available. Especially in a short sale or bank-owned situation, if the bank accepts the offer and takes the property off the market, they need to be sure that the buyer is qualified to move forward.
Getting a loan is not as easy as it once was. A number of my recent clients were sure they would qualify and wanted to to go shopping, but when they contacted the mortgage companies, they didn't qualify. In a cash purchase, the bank needs to know the money is in actual cash - not stocks or other instruments that can take time to turn to cash - not to mention that with an unstable stock market, what is worth a certain amount today may not be worth that much tomorrow!!
I hope this helps answer your question.