A lot of it is. You still want independent confirmation of these things. It reduces incentive to lie (by increasing the probability of being caught), and can mitigate some types of miscommunication. E.g. maybe your ally can't admit something that they know to be true, because it happens to be unpopular domestically.
I'm not sure that this is sufficient justification, but the benefit does exist.
> If this is a necessary element of state diplomacy, how can interactions between countries like the US and Mexico ever be fair?
How can they ever be fair if it isn't? Fairness is just not a property that interactions generally have. Every actor has advantages, and those advantages are rarely equal for all parties.
It's not like diplomacy is done in public, there are ample opportunities to convey your true intentions.
>How can they ever be fair if it isn't?
Sure, you're right that "true" fairness is likely unobtainable. This system adds an unnecessary additional component that serves only to further the aims of the powerful. Calling it a normal, necessary part of diplomacy makes it seem like it's in everyone's best interest.
If this information is beneficial to both sides, it should be freely exchanged. Relying on everybody spying in each other allows poor information to cause massive mistakes, creates a power imbalance should one agency outperform another, and requires a massive investment in a sector that is also capable of economic espionage and the creation of a surveillance state.
If this is a necessary element of state diplomacy, how can interactions between countries like the US and Mexico ever be fair? And how is the public benefitting from this large investment in state controlled espionage?