Here's an example of how I'm using it. Basically to log part of the response body to API requests.

    log_format  main  '$remote_addr - - [$time_local] "$request" '
                      '$status $body_bytes_sent "$host" '
                      '"$http_user_agent" $http_content_length $request_time '
                      '"$resp_body" *$connection $connection_requests';

    server {
        set $resp_body "";
        location ~ "^/api/v1/...$" {
            # Append response to resp_body until it reaches max_len.
            # - ngx.arg[1] is input chunk.
            # - ngx.arg[2] is eof flag (response is done).
            # - ngx.ctx.resp_body holds partial result between calls
            # - ngx.var.resp_body holds final result.
            # From:
            # - https://gist.github.com/morhekil/1ff0e902ed4de2adcb7a
            # - https://github.com/openresty/lua-nginx-module/
            body_filter_by_lua_block {
                local max_len = 256
                local resp_body = (ngx.ctx.resp_body or "")
                if string.len(resp_body) <= max_len then
                    resp_body = resp_body .. string.sub(ngx.arg[1], 1, max_len)
                    ngx.ctx.resp_body = string.sub(resp_body, 1, max_len)
                end
                if ngx.arg[2] then
                    ngx.var.resp_body = ngx.ctx.resp_body
                end
            }
    }

But there are many scenarios where being able to extend the HTTP server via Lua is more convenient than writing a plugin I would think?

I've also used Lua in the past with haproxy and with Redis. It's a powerful, performant, light-weight, and flexible escape hatch/extension mechanism.