If a system is above a certain size or degree of complexity, it's just a matter of time before new code coming out of development efforts will have to be incorporated into the larger system environment. Attention may also need to be paid to interactions with other systems, and any of the implications that are raised in those scenarios. In smaller, less complex systems, this integration may be achievable during development.
In either case, the integration of new (or modified) functionality needs to be tested to assure that it hasn't broken anything, both in the local system and in any other systems that interact with it.
Developers need to know how and where their code fits into the larger system, and thus how to integrate it. As with the Quality Assurance portion of the previous phase, developers also need to know what testing efforts are expected of them, for much the same reasons.