Okay i have not seem somewhere the explanation of why this is important and how does it affect not having it so ill give it a try.
Now it is important to know how does this work without airtime fairness, and to know WHY you always read well turn on the airtime fairness so the 802.11b or 802.11g monopolize the BW. But did you ever ask why? because i did a whlie ago and started searching.
No airtime fairness how it works?
Well let say you got 1 AP 105 ap model which is 802.11n
Connected to it you got 1 802.11g which can transmit theorically to 54mbps
Connected to it you got a 802.11n which can transmit theorically to 300mbs
Now how will this work?
As you know just one client can access the wireless medium at time, if one is transmitting noone else cant, so just one can be on the channel at time ok?
Now without the fairness the AP¨send a packet let say of 1500 to 802.11n station and then it send another packet 1500 packet to the 802.11g station(This is packet based! with no airtimefairness on)(its like a packet level fairness)
This is what how it normally works without the airtimefariness on
Now whats happening here?
if you see 802.11g will need more time int the channel to get those 1500 packet while the 802.11n does not need too much time.
As you see this is awful for the 802.11n!
why? because its making it goes slower, as the 802.11g client spend more time on the channel!! and thats why when you read
"turn on the airtime fairness so slower clients like 802.11b or 802.11g do not monopolize the BW" now you know this it does make sense....
Now what happens when you turn the airtime fairness?
Simple
Instead of sending a 1500 packet for example to the 802.11n and one packet of 1500g, it will assgn a time! for example it will assign i dunno for example 200micro second to the 802.11n station and also 200ms to the 802.11g station so as you see now it "FAIR" the 802.11g will send yes less data than 802.11n but now its not slowering the 802.11n and now the 802.11g is not making the 802.11n going slower.
Thats why its called Airtime Fairness because it give fair time in the air to both clients...
Hope this will be informative for you all thanks for reading :)