You reduce the total operating distance between the AP and the clients.
As you get out of "max speed" range, the AP and client downshift to the next lower speed ... as that range starts to get weak, it'll downshift again .... and so on until you're out of operable range at 1Mbps.
Then you lose the connection.
If you're in a fairly strong signal area, you should be able to kill those rates and not see any significant effect.
If you kill those ranges and users start saying they can't connect, then get a higher gain antenna, or light up those rates again.
Good Luck
Scott