The 19-year-old was surprisingly chosen ahead of Nathan Lyon as the Aussies' slow-bowling option for a pitch that is expected to turn sharply as the match progresses.
Agar's left-arm spin moves the ball away from right-handers, of which England have eight in their XI, and Lehmann believes that could cause the hosts some problems.
He told reporters: "The main reason for the selection is taking the ball away from all their right-handers and we think this is a really important weapon in particular for this Test match on that particular wicket.
"In the tour match Michael [Clarke] felt he had good drift and straightened the ball nicely so that's just the way we have gone with the selectors in this Test match and we're looking forward to him playing really well.
"Nathan Lyon is still in the mix. Obviously there is no reason, apart from the eight right handers that is, why we have gone with Ashton."
Agar recorded figures of 0-24 from his seven overs on a day dominated by the seamers.