First, it'll do more than almost any other measure to restore accountability to British government by ensuring that responsibility for British law-making resides with British law-makers.
Second, and arising out of the first point, the torrent of EU directives and regulations passively implemented by British bureaucrats, mostly without parliamentary oversight, would cease to have legal effect, thus freeing us at a stroke from the micro-regulation of life so beloved of Euro-pols.
Third, we would no longer subsidise corrupt Mediterranean olive farmers or Polish day-care arrangements or whatever else citizens of other countries should themselves be paying for.
Fourth, we would mercifully exit a dangerously (because unaccountable) EU-elite-driven process of political integration which, because much of Europe is actively hostile to it, is likely to end in spilt blood. Continental Europe has given much to the world, but stable governance and peaceful political change are not among them.