Speed, security, and convenience. By e-filing and choosing direct deposit, the IRS expects to issue 9 out of 10 refunds within 21 days, as opposed to 4-6 weeks for a check in the mail.
With a direct-deposited refund, you don't need to worry about your check getting lost in the mail, nor do you have to go to the bank to deposit or cash it.
You can even split a direct-deposited refund into 2 or 3 different accounts.
Please note that due to IRS regulations, you can't direct-deposit your refund into foreign accounts – only domestic accounts located in the United States.
Also, the IRS won't let you deposit more than 3 federal refunds into the same bank account during a given year. If a bank account has already received 3 direct-deposits from the IRS, any additional deposit requests will instead be mailed out as refund checks.