The goals of Assured Access to Space are (PDF of a hearing by the House Committee on Armed Services, page 14):
The existing policy, codified in federal law, requires that assured
access policy and spending, at a minimum, achieve the following two objectives:
- the availability of at least two space launch vehicles (or families of space launch vehicles) capable of delivering into space any payload designated by the Secretary of Defense or the Director of National Intelligence as a national security payload; and,
- a robust space launch infrastructure and industrial base.
In other words: if necessary, the government pays to keep the launch provider from going bankrupt. The benefit to the Government is the knowledge they'll be able to launch their satellites when they need to, rather than having to wait in line with commercial customers, or worse, not be able to launch at all because a link in the supply chain controlled by foreign governments is blocked from delivering critical components.
The program failed in one respect: it continued to rely on critical components from foreign companies (i.e. the Russian RD-180) via waivers.
Assured Access is a common construct in the defence world. Companies that are strategically important are kept afloat instead of being left to fend for themselves on the open market.