It's not necessary, it's just an easy way to get a short-term advantage. It fundamentally degrades our ability to interact peacefully on an international scale, and it is one of the reasons our politicians form a privileged class.
The only justification for it is "they did it first," which is both childish and irresponsible. Just because everyone else does it, doesn't mean it is correct, necessary, or justified.
No. The second use is to "keep them honest". It's not to gain a one time advantage or short term advantage, but it serves as a way to ensure your competition isn't doing monkey business.
It's both an out of band communications channel but also provides a feedback loop. If your enemy or your competitor is engaging in something you all agreed to is out of bounds, you can respond to it with the information you have gained.
EDS and Boeing will know if the other is underselling, bribing, receiving subsidies, etc. and be able to respond accordingly, for example.
Can you tell me what legal framework will allow you to undertake these investigations --to delve into and discover economic secrets? We have enough trouble extraditing criminals, nevermind politicians from sovereign nations. What court makes decisions and who in the court is making those decisions, to whom are they beholden?
See how so many countries are rushing to resolve territorial disputes at the International Court of justice? Only the plaintiffs.
Edit: I'm referring to spying, not torture.