I just thought it would be fun to have a tiny ball of destruction that I could move around arbitrarily. What would I do with it? I dunno... maybe something resembling CNC milling? Etch "hello world" on the inside of a containiner without opening it?

As for building a sensor with it goes... I suppose you could create sources of light very far away without bothering to send an emitter or reflector to that location. Seems like you could use this to build a gravitational wave telescope that was much bigger than the earth.

Probably you could also break some rules regarding line-of-sight communication. If you want to transmit around an inconveniently placed moon you could send an amplitude modulated signal at point on the moon's side, the receiver could send a beam that was nearly at the pair production threshold aimed at the same point. The signal, where it intersected the beam, would take the photon flux over the threshold, repeating your signal from a more advantageous location. Although since we're already invoking godlike technology here... you might as well just use neutrinos to communicate directly through that moon.