The core basic legitimate reason is to verify what is said with what is actually happening; to discover the genuine intentions of the opposition. So, olde days,

The core basic legitimate reason is to verify what is said with what is actually happening; to discover the genuine intentions of the opposition. So, olde days, USSR says "we have no intention of starting WW3", and US spying would seek to verify that. If of both sides can convince themselves that the opposition is being honest, diplomacy can work. If you cant be sure of what is being said, you quickly end up in trouble. Both sides legitimately do this as much as they can. Its a confidence builder. It is why both the US and USSR tolerated spies and had a no assassination rule. They both new, in the olde days, that it was beneficial to both to allow each other to spy and gain confidence in each others position. Trouble comes when nation states extend that to seek political or commercial advantage, or apply it to their own citizens and / allies, who expect to get the benefit of the doubt. Worse still when law enforcement and spy agencies start to combine. This is why I assert that the US, and UK governments treat their citizens as enemies. We have seen plenty of times what its like when intelligence agencies and secret services (SS) are turned on the population. Unless Obama and Camoron do something now to reign this in, some very dark days are ahead of us.