The 2.4 GHz ISM band is fairly congested. With 802.11n, there is the option to double the bandwidth per channel to 40 MHz which results in slightly more than double the data rate. However, when in 2.4 GHz, enabling this option takes up to 82% of the unlicensed band, which in many areas may prove to be in-feasible.
Since enabling 40 Mhz reduces the number of channels that an AP can use this feature has to be manually enabled only if needed as per your requirement.
Hope this helps. All the best.