The amount of energy you would have access to is almost unimaginable. We can only speculate what you'd do with that but two leading ideas are:

1. It may well be the only feasible method of interstellar travel. Accelerating a spacecraft is a whole bunch of fuel you don't have to carry; and

2. Computing power.

So what would motivate people? Living room. One estimate I've seen is the current population of Earth on a full Dyson Swarm would give each person the equivalent living space of the continent of Africa. Of course you'd probably have a ton more people. It's likely you'd have a quadrillion people (>10^15) with each person having significantly more room than they do on Earth.