I would catch one occasionally on television syndication, but without knowing the back story I didn't want to watch it. I also didn't want to shell out the cash to buy each individusl season on DVD. I already have a huge DVD collection and really don't want to add that to it.
Imagine my surprise when I discovered that Netflix has all of the seasons of Star Trek streaming on their site. I can quickly watch all my favorite episodes or I can watch them from the beginning a few episodes at a time. I don't have to wait for them to be on television. I can watch Sisko arrive at DS9 for the first time and the next hour watch as Jadzia dies and is replaced by an even hotter version.
It's not just Star Trek either. There are countless science fiction shows, both contemporary and classic, available streaming. If you want to watch a David Tennent or Tom Baker, Dr. Who, it's there. If you want to watch Sam leap from one person to the other on Quantum Leap. You got it.