Not to split hairs here, but FCC documents indicate that "acceptable" Part-15 transmitters are generally in the milliwatt range. They also state that the signal should not be receivable more than 200-feet away. And you can't hook a transmitter to an antenna with gain that will exceed that limit. So if you can pick up the signal for half a mile around, it ain't legal.
There are many instances where someone may want a little more coverage,
without the intent of being a pirate. Here in the Carolinas there are numerous drive-in movie theaters and churches
that make use of good FM radio transmitters and cover up to 1000 feet in all directions.
The CZH-05B transmitter is a great example of one of these transmitters, it has a high and low power setting
(100mw or 500mw). So it can still be just for your house and not cover more than necessary.
A stronger signal from a transmitter means that every radio in a house or building can be in sync and static free.
It's a better option then running speaker wires/drilling in your house and they can replace lots of expensive stereo equipment.