We find this an interesting question, since TURL.ca pre-exists a large number of today's URL shortening services. Therefore, it's not just another URL shortening service trying to make it through in the big world of social networking. TURL is much simpler than that.
TURL is here because Stephen Olesen decided that learning new technologies, investing time in development of single use web applications and continued development in the .ca domain realm was important.
Back in 2007, when TURL was created, the choices of URL shortening services was extremely small. Those that are interested can take a quick look around Wikipedia or Google to find out which ones were around at that time. Each one of them was just about equivalent, so TURL came into existence as a side project. The intent wasn't to duplicate the exact system and methodology of existing systems, but to provide something that suited the needs of the author. The ability to completely customize the tag returned from the shortener was something rare at the time, and is something that remains useful and handy for a number of users.
As the social networking parts of the Internet started to grow at exponential rates, the needs for URL shortening services became more and more important. When one is limited to very short sentences, making the URL you want to talk about as short as possible was important. This is where TURL started to grow and thrive. As time went on, shorter and shorter domain names came into existence from other providers. We like to think that the .ca portion of our name keeps us distinct and Canadian, letting other Canadian websites keep the .ca branding in the shortened URLs displayed to their users. For the users which aren't Canadian, we appreciate your support of such a simple service based in Canada.
TURL is one of a sequence of projects by Slepp (the author's nickname, for those that aren't aware). The first large production and public project was the Pastebin project, started back in 2004. After the popularity of that service, a large number of other services were spawned, all with the same basic concept — do one thing, and do it well. From there grew a place to store any reasonably sized file for sharing at Filebin, a place to share pictures at Imagebin, the voice message recording service at Voicebin and many others. For the entire project list, you may just want to visit the list of Slepp's projects.