There are many, many uses for a PG signal. One of the most common is to control power-sequencing of circuits with many different rails. For instance, you may have a Supply B that is not supposed to turn on until Supply A is on, and stable. By feeding the PG signal from Supply A to Supply B's enable input (either directly, or through some simple logic gates or a CPLD), they will sequence themselves in the proper order.
Another common usage is for reset supervisor ICs. If such a device has an active-low manual-reset input, OR'ing together all the PGs from supplies (assuming each is open-drain), if any of them fail / go out of regulation, the reset supervisor will toggle a reset of the system.
So it's really up to you, but there's two common uses. You should check the datasheet to see when PG is asserted/de-asserted; for some regulators, if they detect you're out of regulation by 7.5% - 10%, the signal will assert.
In your case, here's the relevant section:
If useful for you as the designer to know if regulation falls out of those ranges, then you should wire it up to whatever you need. Perhaps you want your microcontroller to take an action, or go into reset. Maybe you just want to turn on a LED when PG is not OK. At a minimum, I would bring it out to a test point with a pull-up resistor populated (or at least the footprint) so it's present and accessible if you end up needing it. :)