Trying to Understand Upstream Keepalive

[aanchalj]

As stated in
http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive that
"It should be particularly noted that the keepalive directive does not limit
the total number of connections to upstream servers that an nginx worker
process can open. The connections parameter should be set to a number small
enough to let upstream servers process new incoming connections as well."  I
want to understand if a new client comes, why can't they use existing
keep-alive connections? Do they need to create a new connection with
upstream?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,249924,267553#msg-267553