Ideally, a well-engineered path has just the amount of power required to get from point A to point B with decent reliability. Good engineering limits the signal to the area being served, which results in reduced interference and a more efficient use of the spectrum. Using too much power will cover more area than is needed and may interfere with other users of the band.
Because 802.11 is designed for short-range use in offices and homes, it is limited to very low power.
Section 15.247 of the FCC regulations covers the operation of 802.11b devices.
You are allowed up to 30dBm or 1 watt of Transmitter Power Output (TPO) with a 6dBi antenna, or 36dBm or 4 watts Effective Isotropic Radiated Power (EIRP). The TPO must be reduced 1dB for every dB of antenna gain over 6dBi.
The FCC encourages directional antennas to minimize interference with other users. The FCC is more lenient with point-to-point links: the TPO must be reduced by one-third of a dB, instead of the full dB for point-to-multipoint communication.
More specifically, for every 3dB of antenna gain over a 6dBi antenna, you must reduce the TPO 1dB below one watt. For example, a 24dBi antenna is 18dB over a 6dBi antenna. You would have to lower a one-watt (30dBm) transmitter 6dB (one-third of 18) to 24dBm or one-quarter watt.
Section 15.407 of the FCC regulations covers the operation of 802.11a devices.
As described previously, the U-NII band is chopped into three sections. The "low" band runs from 5.15GHz to 5.25GHz and has a maximum power of 50mW (TPO). This band is meant for in-building use only, as defined by sections 15.407 (d) and (e):
(d) Any U-NII device that operates in the 5.15-5.25 GHz band shall use a transmitting antenna that is an integral part of the device.
(e) Within the 5.15-5.25 GHz band, U-NII devices will be restricted to indoor operations to reduce any potential for harmful interference to co-channel MSS operations.
The "middle" band runs from 5.25GHz to 5.35GHz, with a maximum power limit of 250mW. Finally, the "high" band runs from 5.725GHz to 5.825GHz, and has a maximum transmitter power of 1 watt and an antenna gain of 6dBi/36dBm/4 watts EIRP.
As with 802.11b, the FCC does give some latitude to point-to-point links in 15.407(a)(3). For the 5.725GHz to 5.825GHz band, the FCC allows a TPO of 1 watt and up to a 23dBi gain antenna without requiring reduction of the TPO 1dB for every 1dB of gain over 23dBi.
15.247(b)(3)(ii) does allow the use of any gain antenna for point-to-point operations without reduction of the TPO for the 5.725GHz to 5.825GHz band. Look at the Part your equipment is certified under to see which EIRP restrictions apply.