There are three types of private trackers: general, specialty, and niche. A general tracker has most of the newest of everything - tv and movies and music and games, etc. A specialty tracker focuses on a specific media - movies or comics or audiobooks or TV, etc. And a niche tracker focuses on a specific interest - British television, or horror movies, or dnb music.
A general tracker has very similar content to public trackers, though they tend to be more secure. And like public trackers, while they'll have the latest items, and old popular items, they tend to have retention issues.
Since a specialty tracker has a narrower focus, it tends to have deeper archives for it's content. A movie tracker, for example, instead of having just the most recent movies and a back-catalog of older blockbusters, will have those plus a catalog of older, more obscure, less popular content, and it will often offer that content in multiple formats and sizes.
And a specialty tracker goes even deeper for those that have a particular passion for the subject that's covered.
Do you need a private tracker? IMO, most people don't. Most people are happy with what they have, or are happy with what they get from public trackers and other places. It's really only if you're finding yourself unhappy with public trackers - you're not comfortable with the lack of privacy, for example, or you're often looking content that you can't find - that I would suggest looking into private trackers.