Once joined into a unity, it's the building of "Nerves with Ganglia" that enable communication between the individual cells within a Slime Mold in more complex ways. To the question why would individual cells seek unity, the argument suggests it was a series of accidents that resulted in advantages around such things as food gathering and reproduction. The plan was and is survival in an incomprehensible environment. Have to wonder at the color of the Dog Vomit Slime Mold, bright yellow and fairly disgusting in texture, not something a gardener is anxious to find himself scrapping off the sole of his boot.
In time the Slime Mold may well develop ever more capacities each one of them acquired by accident and judged by each capacities contribution to Slime Mold success rather than its failure. And yet, even if the Slime Mold were to one day open a bank, it would still be a collection of cells, some more specialized than others. And all of those cells would contain memories of simpler times when objectives and purpose were more immediate, more obvious, well written into parts of the Slime Mold that despite the passing of generations linger in a less contemplative yet very determined way. Interesting to consider, whence comes the Question Why. Or is it there already, lurking, primal and full of hope.