My take on the origin system:

Personally, I like the option!
In general because I like to enjoy and impact a great story.
I love experiencing the companion stories, and even more so when I can play as one of them.

I usually enjoy playing a wizard, but if I create my own custom wizard that makes Gale pretty useless as we're limited to a party of 4. So if I can play as Gale, it means I get to know his story as well as 3 other companion stories which will make my playthrough much more interesting.

For the same reason I read books and watch movies, I like to enjoy a good story. And I'm pretty sure Larian's stories are better than whatever I can make up, as my imagination is lacking.

Now I fully understand the appeal of a custom character, and do enjoy creating my own char and being the hero of my own story (especially in multiplayer), but being able to experience the origin stories first hand will definitely add replayability for me.

I think it's a great way of experiencing an NPC if you plan to play the same class, so I don't end up leaving them in camp (and yes, of course you can play with a full party of mages/priests/warriors/... but that takes away a lot of the fun of classes).