For some reason I assumed every character started out with 10 Obligation and didn't "get" anything for it. It was just there.

But I see prefab characters, like the ones from Free RPG Day that have just 5, or Sasha the Explorer from the Beginner Game that would have to incur +10 just to have her stats, and I don't know what the Base Default is. I see suggestions for starting values, but do you "get anything" for it? Is there some minimum that just exists without any benefit whatsoever, or do you get some kind of Spiff for every 5 points above zero?

You start with a minimum suggested Obligation depending on your group size (so the free RPG day module, which assumed there would be 6 players, had everyone at 5). GMs are free to increase/decrease this at the start of the game, of course. You don't really "get anything" for this starting obligation. Or rather, you "get" to play your character

Any obligation taken over and above your starting obligation can be used to gain extra credits or XP at character creation. You can take extra obligation up to your original starting value.

This is all detailed in Chapter II of the Core Rules.

The premade characters don't seem to follow Core rules.

Look at their Talents, the ones they can buy are way down the tree.

The base Obligation is determined by the party size. The more players the lower the base Obligation is. Look at the chart on page 40 (table 2-2)

The premade characters don't seem to follow Core rules.

Look at their Talents, the ones they can buy are way down the tree.

There is a sentence in the Free RPG day module that points to premade characters actually being "established" characters. It's not their first adventures. I wouldn't be surprised if they began as party of PCs that the devs rolled up themselves for playtesting purposes.

The premade characters don't seem to follow Core rules.

Look at their Talents, the ones they can buy are way down the tree.

Pretty much this.

Them being "established" is likely a handwave for the fact that they didn't follow the standard character generation rules.