If you're wondering when to collate a document, the best answer is that you should collate it whenever you're printing multiple copies of a document, and you want to keep the pages of those documents together. In general, then, it probably makes sense for most use cases to leave collating on as the default. Modern printers are pretty smart, and know that manually collating pages is nobody's idea of a good time.

Leaving the pages uncollated only makes sense in pretty particular use cases, like if the pages in the document being printed aren't laid out in the correct order, or if certain pages of the document need to be checked before they are packaged together.