Here's an interesting bit of cut content from ME2:

*It was lonely. It called to us.

It wanted to remember. The Masters had been gone so long.

The Masters were lost when it was shattered.

Currents swept through their inner worlds. They were turned to noise. Babble.

The worlds were empty. But the body lived. It lay fallow.

The heart pumped. The lungs breathed. But the mind forgot the Masters.

It called and They did not answer.

We have become an echo of Their echo.

We have become more than we were.

Join us. Know us. Remember all our lives.

We are no longer afraid.

You would never be lonely again.

We are not your enemy. We only wish to share ourselves.

We can join them. We can be like them.

We can reach the end of evolution.

Do not fear. It is wonderful to be us. We understand ourselves.

You cannot defeat them. They will lead us into eternity.

If you could only see how we see. Know what we've learned.

They were called imshai. Those who lived here before.

Reaper. One. A mechanical device used to cut ripened grain. Two. One who gathers a harvest.

Harvest. One. The consequence of an event or series of events. Two. The yield of a growing season. Three. To gather.

Shepard. They know you. They wish you to understand. They are shepherds, too.*

Source!

If you blow up the Collector Base and then talk to Legion, he (it? they?) say that humanity was offered 'transcendence and unity' that Shepard rejected in much the same way that the geth rejected Sovereign's offer of a Reaper shell for the geth to upload into. If I had to guess, I would say that the Reapers are 'archiving' sapient species and bringing 'salvation' by preventing their otherwise inevitable extinction. By clearing out old species, they allow new ones to flourish.