Because your router is a dual simultaneous broadcasting router. So it broadcasts both 2.4 and 5Ghz bands on separate SSIDs at the same time.
Reason for some devices only seeing the different broadcasts is that some devices do not support or were developed to support 5Ghz or N mode WiFi signals.
You can use either on a daily basis. Some people can watch movies or gaming over WiFi using 5Ghz. 5Ghz has some drawbacks as it is susceptible to interference by buildings, walls and doors. Just the nature of this high frequency and is better in line of sight operation or maybe on wall away.
Most people seem to use 2.4 for general use.
I recommend using only what your devices need. If you don't have any devices that use A or B modes, then switch to single G or Mixed G and N.