Dream Harvesting

The reasons I can think of there being on why some force would harvest dreams is because of the power involved with shaping worlds and beings into existence. If such power can be harnessed through something so simple as dreaming which presumably every sentient being is capable of, then with the right way of directing that energy the power can be used for many purposes.

Though many worlds/demi-planes are created by dreamers all the time, it might be someone's purpose to create a new fully fledged plane by using these dreamers they gathered together. From the more they dream the bigger the plane becomes, until some threshold is pushed and the plane becomes stable. The reasons for doing this could be to replace another plane, or as an experiment, or for some sinister purpose like establishing a beachhead on the Multiverse to invade it. That last point leads to the next point.

Since beings can be created by dreamers, and it's very likely that the Quori themselves are possibly the creation of one very powerful dreamer, then another purpose would be to use the dreamers to create an army. Usually the problem of such an army is that it could only exist in a world created by dreamers, but anyone who could direct dreamers into building an army could find a way for their creations to last outside that world. Maybe they're permanent, but with an army of dream-spawned shock-troops they only need to last a few hours outside the world they were created in.

And the final thought relating to the Quori, is that maybe Quori are not well-known across the planes because they are only dreams of a very powerful ancient sleeping somewhere in Eberron like one of the Rakshasa Rajahs or a Dragon sealed away since they're Age of Demons. And from where they are in Dal Quor they aim to expand their reach. Already the Quori have the ability to one particular world based on them psionicly influencing people on Eberron, and building the nation of Riedra centred around themselves as the Inspired. But rather than just relying on that one theorized dreamer, they decide to have they're Inspired vessels planewalk across the multiverse, and start collecting dreamers for their plan of expanding Dal Quor and keeping the Age of the Dreaming Dark. In some cases they're plans may backfire on one captured dreamer, and that dreamer may escape or become self-aware working against their plans. They may either create a rebel Quori who's opposed to their plans spawning a new line of Kalashtar across the planes, or that dreamer who awakes may somehow become a Kalashtar themselves.