While watching the video tutorial for the Challenge Phase of AGoT they give an example of Player 1 using Catelyn Stark to issue an Intrigue Challenge vs. Player 2 who decides to use Cersei's Attendant to defend. It doesn't seem to make much sense since it's obvious that Cersei's Attendant doesn't have sufficient strength to win the challenge. Just curious why someone might do that?

Because there are some effect that can be activated only if you win an unopposed challenge (for ecample assault of the kraken) so to prevent that you opose a challenge also if you know you will loose. Moreover if you let a challenge unopposed your opponent gain one power.

There are several reasons. Gualdo has touched on a couple, but just to reiterate and add my own:

• Prevent unopposed: Unless your opponent has a lot of characters with renown, they probably will need to win unopposed challenges to get enough power (15 needed) to win the game.
• Prevent opponent's "response" effects: All kinds of responses work off of winning challenges unopposed or winning by X strength. One common card is Seductive Promise, but that doesn't really apply in this example, because that can be played only after a power challenge. (That gives you an idea though.)
• Other random effects: There are a variety of effects that can be used only when your characters are knelt. For example, To Be A Lion (this is the Lanni version, but each house has a "To Be A XXX") is playable only if you have a knelt character that you can stand to pay the "cost." (I know standing a character is more of an advantage than a cost, but in those situations where you don't have a knelt character, you can't play it.) Other effects might make your knelt characters stronger, etc. (I can't think of any off the top of my head, and usually you do want your characters standing, but this just gives you an idea.)

In all likelihood, #1 is the only reason that applies, since there are generally fewer unopposed responses in Stark and few responses to winning by X against intrigue challenges. But this gives you a general idea why some opponents will block even if they know they will lose.

Excellent guys, thanks. I'm a total novice to the game so if my question seemed obvious it's only because I don't have much of a grasp on the nuances yet.

Twn2dn said:

An example is Ser Dontos Hollard (Tales from the Red Keep, F62), which was featured in a recent Card of the Week article. That card's ability can only be triggered by standing it.

Also, don't forget that you can boost a character's strength after it's in the challenge with cards like Rusted Sword (granted, Rusted Sword by itself wouldn't be enough to win, but again, you get the idea).