Self-signed certificate doubt

Hi all,


I'm following the step of README file (WSD/README) to generate the new self-signed certificate. 
Could anyone explain me what's the 3 last commands of this list means?


> openssl genrsa -out key.pem 2048

> openssl req -sha256 -new -key key.pem -out csr.pem

> openssl ca -keyfile private/ca.key.pem -cert ca.cert.pem -extensions usr_cert -notext -md sha256 
> -in csr.pem -out cert.pem


I understand that it's generated a private key then the CSR certificate is created and next it's 
created CA certificate but it's already created in the previous steps. Why do this again?


Thanks in advance.

Besta Regards