I dare say the main reason to use it is for the wifi file transfer/sever management aspect (just recently SAMBA got ported which should make windows access a whole lot easier) but there are a whole mess of protocols that are supported.
On top of this you can run scripts (think batch files) to do nice things on your DS file system (there are two other apps I know of DSOrganize and DSBrowser and while they are good they can barely touch DSlinux for this.
In all truth though there are standalone applications that can do the rest better (retawq is OK but stuff like Costello's DSHobro and the DSOrganize browser
Email: DSmail
Google Earth: Treasures of Gaia
IRC: CLIRC and DSOrganize (try the former)
Other chat apps are also done.
Media can be taken by moonshell and lmg-ng
If you do not know linux or that much about PCs I would suggest a liveCD (ubuntu/kubuntu tend to be the choice ones or taking a bit from a partition to do it in which case I have been playing with mint (based on ubuntu but eschews some of the more irksome design philosophies in favour of creating a more friendly, manageable environment):
http://linuxmint.com/ for while the DS version is great it is somewhat limited