If you use a wireless AP you should use "regulatory domain" and select correct country to respect local regulation
And not change anything else.
If you use external antennas/PtP links etc. you want to play with the output power at modulation rates.
With 17dBm max output power you can not do much (I assume this is a hap lite or small AP product integrated antennas).
With such a product you can lower the output power to reduce interference if you have couple of Wifi AP close buy.
Or say on AP that only needs to cover a conference room, or reduce radio "smog"
Playing with output power makes more sense / is required with high power devices where indeed you might not need all the power
or you have high or low gain antenna that you want/need to match over data rates.
PS: If the HW provides you max 17dB, you can put in 35dB, but it won't change anything and reset to the 17 value
PSS: If the client does only have ~17dB output power on his side (such as phones , tablets, which have even less in high data rates) there is
no need/benefit to further increase AP output power, as the client won't be able to reach back to the AP over distance.
In that case only increased receive sensitivity will help, via for instance 4 chain Wifi radios... (such as RB4011!), beam forming etc.
