The importance of shaping your own future, and not just having it handed to you, is a running theme in the series which you can see in a few places like Mordin's discussions of the Krogan uplifting (and anything else related to the Krogan really) and the geth's desire for self determination. This is important because of the way it relates to the reapers, since their plan is based on giving incredibly advanced technology to younger civilizations in order to control the way they develop and make the cycle more effective. The relay network has been a huge beneficial to the galaxy, but it was ultimately a death trap. Destroying the relay network at the end of the series is meant to show how the galaxy hasn't just turned back the reaper invasion, but defeated the entire cycle, and how it's now their turn to step out from the ruins of their civilization to create their own future instead of being stuck in the world the reapers created for them. Those explosions bring more than just destruction, they bring freedom.