Some decades ago, the FCC used to require that US amateurs log every single QSO. And those logging requirements could be somewhat complex, especially for third-party traffic with other countries.
It's the law because those 5 channels are shared with other licensed commercial services. If we interfere with those, then the FCC might pay us a personal visit and see what our ERP is and what it might have been with our 'previous' 60m antenna (indicated, of course, by our log).
Amateurs may transmit with an effective radiated power of 100 W or less, relative to a half-wave dipole. If you’re using a commercial directional antenna, FCC Rules require you to keep a copy of the manufacturer’s gain specifications in your station records. If you built the directional antenna yourself, you must calculate the gain and keep the results in your station records.
When using a directional antenna, you must take your antenna gain into account when setting your RF output power. For example, if your antenna offers 3 dB gain, your maximum legal output power on 60 meters should be no more than 50 W (50 W plus 3 dB gain equals 100 W Effective Radiated Power).
It's intended to make sure that the ERP does not exceed those limits.
From http://www.arrl.org/60-meter-faq#:
How can I be sure I don't exceed the power limit? The FCC said hams may run 100 W effective radiated power (ERP) on the five 60-meter channels. The new rules say, "For the purpose of computing ERP, the transmitter PEP (peak envelope power) will be multiplied by the antenna gain relative to a dipole or the equivalent calculation in decibels. A half-wave dipole antenna will be presumed to have a gain of 0 dBd." This means if you use a half-wave dipole (about 87 feet 3 inches for the "middle" channel according to the formula), set your transmitter's output power for 100 W PEP (many transceivers' meters can be set to indicate peaks), and you should be in compliance. The FCC asks licensees using antennas other than half-wave dipoles to "maintain in their station records either manufacturer data on the antenna gain or calculations of the antenna gain." This is a new record-keeping requirement for amateurs. The "best" antenna configurations are those with a proven track record on the lower bands, keeping in mind that using a loop or an array of some kind will require you to "do the math" to ensure you are not radiating more than 100 W ERP in any direction. For the math you must reduce your power by the number of decibels your antenna gain exceeds 0 dBd (0 dB relative to a half-wave dipole). Conversely, you can increase your transmitter power if your antenna exhibits loss compared to a dipole. Be prepared to document these situations in your station records.
