Why do I have 2 home wifi networks? One is 2.4 GHz and one is 5.0 GHz.
5 GHz has greater throughput, but is pretty much line-of-sight... It has a very short range and gets blocked by walls.
2.4 Ghz has a greater range, but the through-put is not as high.
Having both allows you to connect certain devices to certain wireless networks depending on their location in your home and what they are being used for.
Another reason is that the USA can use the 5 GHz band for internet data, but folks up north in Canada cannot. All the router and wireless printer manufacturers try to build one single device that can be sold to everyone in North America.