Sometimes these articles pretty much write themselves.
Oh, you’re still here? You want more explanation? OK, I respect that.
Every antenna has an innate capability to pick up a signal, called “gain.” Gain is actually measured as how much better a particular antenna picks up signal compared to a reference item. You can get more gain two different ways. One is by having a bigger antenna. The other is by using an amplifier. This means that you can have this massive outdoor antenna with 20dB gain, and this tiny indoor antenna with 20dB gain, which sounds like they do the same thing. Except, well, they don’t.
If you’re not using an amplifier and you’re getting 20dB gain, you’re actually pulling in more of the actual broadcast signal. You’re actually pulling in precisely 100 times more than you could pull in without that antenna. If you’re using an amplifier to get that 20dB gain number, you’re still getting a very weak signal, you’re just making it 100 times louder.
Here’s a way to imagine the difference. If a person is speaking clearly and loudly, then you can hear them. If they are mumbling and mushing together their words, even if you record their mumbling and make it really loud you still aren’t going to understand them as well as if they were actually speaking clearly in the first place. Maybe you’ll understand them well enough, maybe not. The clear speaking person, that’s what you get from a real, honest, big antenna on the roof. The mumbler, that’s your amplified indoor antenna.
Now I’m not bashing indoor antennas. When used within about 25 miles of the towers they have enough gain to get the job done. But 90 miles? Forget about it. If they pull in any signal at all, it’s going to be so weak that no amount of amplification is going to make it usable.