Why not? That's like asking why would you lock your important documents in a desk drawer at your house. Some people do, some people don't. I'll encrypt where I can encrypt.
Also highly recommend Carbon Copy Cloner. Don't just have one backup. I personally use BackBlaze, TimeMachine, and have everything in iCloud. I also used to CCC to an external disk up until very recently (gave the disk away (after formatting it)). 😛