These are animals that have backbones. But what was the first kind of backbone? Why is having a backbone a good idea?

From wikipedia

Vertebrates originated about 535 million years ago during the Cambrian explosion, which saw a rise in organism diversity. The earliest known vertebrate is believed to be Zhongjianichthys. One of many early vertebrates are Haikouichthys ercaicunensis. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tai