I navigated to this definition:

computronium:

Matter that has been transformed from its natural state into an optimized, maximally efficient computer. (A true Extropian would argue that this is matter's "natural state".)

What constitutes "computronium" varies with the level of postulated technology. A rod logic nanocomputer is probably too primitive to qualify as computronium, since large molecular aggregates (hundreds or thousands of atoms) are used as computing elements. A more archetypal computronium would be a three-dimensional cellular automaton which attached computational significance to each individual atom, perhaps with quantum-computing elements included.

More exotic forms of computronium include neutronium, Higgsium, monopolium, or - my personal invention - an interlaced structure of positive-matter and negative-matter monopolium wrapped up in a fractal Van Den Broeck warp. (The total mass is zero, so the whole doesn't undergo gravitational collapse. If paired negative and positive matter can be manufactured in unlimited quantities, the fractal Van Den Broeck warp can continue extending indefinitely and exponentially. Threading the system with wormholes keeps latency down. And the whole thing fits in your pocket.)

This stuff must rank in the more amazing achievements of human imagination. How someone can come up with this, presumably not under the influence of any substance (it's quite logical in its way) astounds me.

Now, you asked:

Why have ships full of needy biologicals roaming the galaxy when you can have an AI do the same with much more efficiency?

and clearly the answer is that a species capable of this sort of imaginative reach would not be satisfied with sending emissaries and substitutes ..

In other words: because WE WANT TO GO THERE OURSELVES, MAN!