It comes handy when you need to do that odd symbolic computation which is too big to handle by hand (or you are plain lazy).
By looking at the wikipedia entry, I discovered an startling truth: as of 2009, yacas is no longer maintained.
I should have guessed, because yacas does not compile out of the box (the program needed to generate the manuals does not compile), and this led to some naïve patching (just removing all instances of said program...).
The compilation process for the NanoNote with OpenWRT follows the usual lines, with a twist. Yacas uses a host program to generate code, named mkfastprimes. And when cross-compiling, the resulting executable cannot be used.
To solve this small problem, you need to compile yacas in your host computer, and use the generated mkfastprimes to get the desired files. To do so, in the makefile I download the package, uncompress it, configure and make. Some minor problem is that configure generates several makefiles and folders in the working directory... and looks like OpenWRT's makefile system does not allow to change it. Thus I copy the original Makefile to Makefile2, let configure generate a new one, make yacas and copy back the original Makefile.
And this can get you yacas working in the NanoNote. You can download the Makefile and patches from here.
For additional instructions on how to set up a build environment, please refer to Qi hardware's wiki here.
Gcal: the ultra-powerful command line GNU calendar
Unpacking my Ben NanoNote
My first port to the Ben NanoNote: gnugo
Another Ben Nanonote port: pmars