By default when Nginx starts receiving a response from a FastCGI backend (such as PHP-FPM) it will buffer the response in memory before delivering it to the client. Any response larger than the set buffer size is saved to a temporary file on disk. This process is also explained at the Nginx ngx_http_fastcgi_module page document page.
Since disk is slow and memory is fast the aim is to get as many FastCGI responses passing through memory only. On the flip side we don't want to set an excessively large buffer as they are created and sized on a per request basis (i.e. it's not shared memory).
The related Nginx options are: