[unfamiliar]

I rely heavily on spotlight to find files I want, and about 99% of the time it works perfectly. Sometimes there is a file I am going to keep that I suspect will be hard to track down using spotlight in the future; either because it has to be badly named (source code files), will not be easily indexible (scanned pdfs or images), or my interest in it is not directly connected to the content of the document (i.e. a description of an algorithm with an obscure name which I have a specific application in mind for).

In these cases I use the tags feature to augment the metadata so that I am pretty confident I will be able to find it quickly in future, even if I can't remember exactly what it was or why it was originally saved.