I needed it to receive WUNC reliably. I'd get it dialed in, then a couple of days (or weeks) later it would get dodgy. Tweaking the antenna fixed it for a while. I started out with the amp only on the WUNC leg, but moved it over to amplify both antennas when the networks started having issues with macroblocking. The signal went to crap a couple weeks ago, and I thought I might have a bad wire. But it turned out to be overmodulation from the amp. Turning the amp down fixed it.
I tried to avoid buying an amp for a while, because I'm a cheap ass. But now that I have it, I like the adjustability that it gives. Figure that the signal is affected by solar radiation (which varies), so that factors in as well. Maybe I tuned the antenna at night, but the kids couldn't watch PBS during the day. Who knows.
Also, keep in mind that the signal needs line of sight. The higher you can get the antenna, the fewer obstructions that are in its path, the better the signal will be.