In very dense deployments, especially when wireless applications have high bandwidth requirements it is useful to have several closely placed access points providing service to wireless clients.
Reducing the maximum transmit power of the radio would help keep the APs from overlapping too much on each other, ensuring that clients are serviced only by the nearest APs with the strongest signals.
In addition it also helps to disable the lower data-rates/modulations as these tend to have much higher 'reach' than the higher data-rates. This way once clients start to move out of the range of an AP at a moderate data-rate, instead of continuing to maintain a connection with increasingly lower modulations, the client can instead roam to another AP.
Lower rates also mean more airtime usage for the same transmission so removing them configuration improves effciency.
To disable the rates set Configure-> Radio >Minimum Unicast Rate configuration under the appropriate radio, and choose a value like 12Mbps:
Note that disabling lower rates on the 2.4GHz radio has the additional side-effect of ensuring that clients that only support rates like 1/2/5.5/11 (old 11b clients) can now no longer associate with the radio.
Depending on the types of clients being serviced this can be another added benefit. For instance in a free open hotspot or guest network such low-speed client devices would have impacted other high speed clients and reduce overall cell efficiency.