It de-risks and accelerates the transaction. Most mortgage based purchasing requires the loan lender to approve the house for the value (even if you are 'pre-approved' for a loan of $X, if the bank doesn't think the house is worth $X it won't let you take the loan out for it). All cash offers don't have those approvals or risks, however slight, and also have no/less time delay - you can close in a week (just a title/deed history search, generally).