Why did President Franklin Roosevelt encourage the passage of the Lend-Lease Act?
The British were in pretty desperate at that point and needed US support. Roosevelt was trying to assist them, as he thought a war was coming, without offending the Isolationists.
The cost/benefit of the deal to the US was substantial in terms of area gained for bases and pretty low cost to the US because the destroyers were of limited value.
As a sideline about this time British intelligence secretly gave the US a lot of technology and intelligence that they would have been otherwise unaware. This include Enigma that was used by both the Germans and the Japanese, and nuclear research among lots of other stuff.