It isn't so clear there is a reason for the crime of 'attempt', or what or how good the reason is. The Star Chamber, which cooked it up, is universally condemned as oppressive. You might say they anticipated science fiction tyranny, when they invented the world's first thought-crime: they were able to make a crime of 'conspiracy' because they imagined that the speech of the conspirators was an 'outward act'; inevitably its glorious future was e.g. to jail the left wingers in the McCarthy period.

The point of view that says 'there is a reason we punish attempts' has difficulty explaining why we punish 'success' _even more_. There is a sort of bad concience about it. The paradoxes are discussed in a characteristically brilliant and twisted work 'The Punishment that Leaves Something to Chance' by David Lewis, one of the greatest philosophers of the 20th c. It nominally defends the law of attempt but can as well be read as a catastophically destructive parody of it. https://andrewmbailey.com/dkl/Punishment_Chance.pdf