They're used for when you do things that multiple people can foil, or can be foiled at a later time. Hiding so that all those guards passing won't see you (Your Stealth check is their TN), making a bomb that's difficult to disarm (Your Demolitions check is the TN), designing a contract that's difficult to understand (Your Law check is the TN), and stuff like that.

You never know who's going to try and get past your stuff, and having you make another Security Systems check every time someone tries to sneak past your camera and tripwire setup can not only get annoying, it can tip the GMs hand (Roll Security Systems? Someone must be breaking into my apartment on the other side of town!).

It's easier just to have them roll at whatever you got when you setup the system and have whomever trys to get past it roll at that TN.

Edit: I looked up for #2 and yeah, you're probably right. It's supposed to be +Karma for the agressor.

I suppose because the interrogator/intimitator can't really "lose" the contest. The target may resist the effect; but, nothing really happens to the interrigator, and the target may have to roll again as more time passes (Say he has to roll every hour and make a success or risk spilling his guts).

In the case of intimidation and interrogation you're not trying activly accomplish something. You're trying to create an environment and set of circumstances that thwarts someone else. It's up to them to resist the effects you place against them.

Of course you could use that logic to rationalize that a target must make the open test and the interrogator/intimadator should make the regular test.

Or you could just make an opposed test. The agressor rolls Intimidation agaist the targets Willpower and the target rolls Willpower against the targets Charisma (Or perhaps Strength, depending on how he's doing it).

It's up to the players and GM really. The way that works best is the way that works best for you.

It's used to try and generate your attempt to achieve "your best". The more dice, the better your chance to score higher on an Open Test (OT). I use the OT when pitting two things against each other where only one success would be needed to show a victor. I've noticed that when someone is "trying their best" it's rather abstract to assign a TN and make a typical Success Test (ST).

Like stealth for example, the OT generates the TN for anyone that would need to make a perception test to see them. Switching this around as a ST, what would the TN be? What would the successes measure? Many folks use a TN=4 and observers use TN=4+successes to detect them. This might work, but you set a limit = TN+Skill dice as the maximum TN for an observer.

On the other hand, OT results are only limited by the number of 6-rerolls that are made. The higher your skill, the better your chance to get a high TN.

Everyone's got their own take on OT vs. ST and many folks simply remove it. Although not glaringly obvious it has it's place IMO.