To understand this , we have to first know what Is a Web Server.
A Web Server is a computer that stores all the data of a website like you see those images , texts , videos all are stored in the web server. Every web server has a limit of the number of people who can visit the site at a time.
If the limit exceeds, then the website crashes. So , many companies use (load balancers) multiple web servers to balance such load, these servers are called www1 , www2 etc. When www is busy and takes time to load , we are automatically redirected to another server www1.
That was the reason we see some websites have www1 instead of www.