The leaks revealed that the Pentagon had failed to list tens of thousands of civilian casualties, which inspired the Iraq leadership to insist that U.S. troops not be immune to Iraqi law, which was a deal-killer for Obama's plan to keep combat troops in the country.
It certainly wasn't "abundantly clear at the time" that Hillary Clinton ordered American diplomats to spy on U.N. officials (which would be a crime for a normal person).
It wasn't clear that Obama worked with the GOP to kill a probe into the Bush Administration's torture program.
It wasn't clear that the Obama administration had pressured Germany not to prosecute CIA officers for torture and rendition.
It wasn't clear that the U.S. had pressured the president of Yemen to lie about U.S. airstrikes in that country which killed many civilians.
It wasn't clear that the Obama administration lied about shipping weapons to Saudi Arabia for use in Yemen.
It wasn't clear that the UK had given assurances to the U.S. that it would limit the scope of any investigations in the Iraq war.
A few of these things might have been suspected, but Wikileaks provided direct evidence.