Power output limits stated in terms of EIRP can be quite helpful in the case of very low frequencies, where it is difficult to build an efficient antenna and at very high frequencies where feedline and dielectric losses become significant. In these cases, a higher transmitter power than may otherwise be allowed can be deployed to make up for these systemic losses.
A 24 dBi antenna provides a gain of >251 times that of an isotropic antenna. This means that if you put in 100 mW into this antenna, your EIRP will be >25 watts - well over the 100 mW EIRP legal level. To use this antenna legally, the power going into the antenna can be no more than ~0.4 mW. This is probably rarely followed in practice. Do take care as 25 watts at this frequency can be harmful to body tissues at close range.
Assuming the directivity of the antenna favors the application, a high gain antenna offers the following advantages:
- Increases the received signal by the same gain
- Reduces receive interference from other (low gain) directions
- Makes up for feedline loss on transmit and receive
- Focuses the transmit power in the desired direction reducing interference to other receivers
- May reduce the odds of reception of transmitted signals for nefarious purposes
The potential disadvantages are:
- Mechanically more complex
- A larger antenna profile
- May not be applicable for a wide area system if gain direction is not favorable to the required coverage area
- Potentially higher cost
If you require coverage of a larger area and you wish to use high gain antennas you could consider multiple transceivers and multiple antennas aimed in different directions.