what's the purpose of multiple channels on Bluetooth Low Energy
If there were just two channels (one for uplink, one for downlink) you could only connect to one device at a time, and you could not operate multiple Bluetooth Low Energy devices simultaneously (unless some arbitration system was implemented where devices would take turns at transmitting at the expense of data rate).  

Multiple channels not only permit multiple BLE links to operate concurrently in the same area, they also enable the use of frequency hopping, where all BLE devices rapidly change channels in a predetermined pattern (direct sequence spread spectrum). This rapid switching of channels makes it difficult for any single source of interference to jam any of the data links.