Sometimes it’s preferable to watch a TV series (for the first time) once the show is over because you can watch it on your own schedule. You don’t have to wait every week for a new episode and you don’t have to wait for the new season to begin every year. And you don’t have to try and recall what had happened previously (because it’s easy to forget characters and events the longer a series is). It’s nice that you don’t lose the momentum of the story when you can watch episodes continuously.
On the downside you’re behind everyone who has already watched the series, and while it may be shiny and new to you, they’ve moved onto other things. And any shocking moments of the series may be spoiled if you don’t watch the show right as it comes out because people like to talk about their TV shows. Certain shows are popular enough that it’s hard to avoid having any foreknowledge about it before you finally watch the show belatedly.
It’s a dangerous thing to have so many episodes at your fingertips because it’s easy to binge-watch and watch episode after episode. It’s addicting, especially when there are cliffhangers egging you on to watch the next episode. It’s easy to get sucked into a good story and hours are quickly lost. I know that binge watching isn’t really the best use of my time, but sometimes a good show is just too hard to resist.