I don't know what the Unclump button on the Align and Distribute tab does. After experimenting, it seems to just move objects around. Can someone can explain to me what it does?
From the Inkscape release notes that I found with a simple Google Search:
The new Unclump button attempts to reduce the local unevenness in distribution of the tiles. When you unclump a tiling, each tile tries to move to a point equidistant from its closest neighbors. If a single unclumping is not enough, you can press Unclump repeatedly, trying to achieve a balance between eliminating small-scale clumps and preserving large-scale features of the tiling. Unclumping can be applied to both randomized and regular tilings, changing them both into a characteristic texture which appears random, but not blindly random — very similar to what a human would produce if asked to evenly fill a space with random dots. As a result, properly unclumped dot tilings remind of hand-made engravings.
In short: move objects to more evenly space the edge-to-edge distances, as mentioned in the Inkscape Manual