Much renovation and DIY work involves starting with very basic materials -- wood, metal, fastenings and adhesives -- and forming them into new objects. Starting from the basics is always a useful skill, but you should also be ready to recognise when using something pre-made will let you get a better end result.
I wandered down this train of thought after seeing a hack featured on Lifehacker favourite IKEA Hackers which involved making a headboard display from a number of bog-standard Lack tables. Given that this essentially involves mounting and decorating three squares of wood, you might think that repurposing a table isn't the most sensible or economic way to go about it.
It is likely the case that you could get similar amounts of plain pineboard or the like from your local hardware store and pay less than the $9.99 a piece you'll pay for a Lack table. However, the potential savings are only part of the story. (In this particular example, the tables were actually on sale, but that's not necessarily something you can assume.)
Firstly, while the basic wood might be cheaper, you'd have to put more effort into cutting it to shape and finishing it than is needed with the tables themselves. That isn't necessarily the best use of your time, and it also might not match up with your skills. (I could do a much better job of mounting wood to a wall than cutting it to size.)
Secondly, it's entirely possible that the idea wouldn't even pop into your head if you just thought "I need some kind of headboard". The table tops themselves are part of the design inspiration -- a concept that only came into being because of them.
This certainly isn't an absolute principle you need to apply all the time. If you've got serious design skills and plenty of hardware competence, then you'll be able to make whatever you want from scratch. If you're less confident or less practiced, then starting with what you can find in the IKEA As-Is section or during the local council cleanout can help.
Got your own examples of where you repurposed an object to a useful DIY end? Tell us about it in the comments.
Lack Table Head board [IKEA Hackers]
Lifehacker's weekly Reno 101 column covers the basics of renovation and DIY.