Remote control for your downloads.
a2web is written in C and is licensed under the GPLv3.
It was born out of the observation that every existing webUI required PHP, no matter which client.
With aria2 itself being light, having to run PHP to be able to control it remotely is a fairly pointless.
With a matching webUI, one should be able to have a decent experience from more restricted hardware, such as the common router, without wasting RAM and cycles for something as big as PHP.
If the webUI was the only user of PHP, having to run the full PHP stack comes with the other usual downsides: wider attack surface, and another service to configure and worry about.