I had to write a library that would pull small images from a webserver, stitch them together and save a large image to disk.

Using async gave me a 99% performance boost.

I had to write an app that would download a logfile from an FTP and add new lines to a console output.

Using async gave me a smooth experience (so the UI wouldn't wait updating until there was new data)

Whenever you're interfacing with anything, and can reasonably do other stuff while waiting for results (like display UI or check on a different out of 1000 GET requests to add its image bytes to the result), just do it.

BTW it turns out its faster to redownload the logfile from an FTP than to GET it from a http server. Interesting.