What is the purpose of sub netting, and how in layman‘s terms does it work?

So I understand the basics of how to conjure up these subnets and the absolute basics of what they are. Class A, B, C subnets signify the number of octets that are given to the network park of the address.

But I don’t understand fully why they exist. Say we have a subnet for your house of 255.255.255.0. What is the significance of this thing and what exactly does it do for you? As I understand it, the first three octets are the network portion. The last portion is the host address. 192.168.1.11, 192.168.1.12 and those are apparently two computers on the exact same network, with different host numbers 11 and 12. But I’m having trouble understanding what the point of sub netting is or what it actually does?