A lot of people (especially me) are not content with 'out of the box' and often consider 'converting' our kits, using other kits, after market parts (in a similar sense to car parts) or even go as far as manufacturing parts. There are a number of reasons we do this (aside from being fun!).
To represent non-existent kits
To make a cooler looking unit
The 'rule of cool' is often a good reason to convert a model or unit. Whilst some will be happy with a standard kit, some feel that this isn't good enough, or that it doesn't quite fit with the style you've gone for. I could have easily used a box of regular chaos marines to represent my chosen, but it didn't look rihgt. Instead, a box of Grey knights was used, for their stylised armour and cool halberds, fitting weapons for a sorcerer's bodyguard. As well as this, I've recently finished a chaos Sorcerer on a disc, using the new finecast kit. Pictures will most certainly follow, so stay tuned.
To make a nice centrepiece
Every army deserves a centrepiece: something which makes the army stand out, and best represents the appearance, design and function of the army. It can often be a bold hero or a battletank. For me, the central feature is my Land Raider. A simple conversion of the tracks and some ForgeWorld doors makes it stand out greatly from the 'standard' examples