Currently, the NSA (and others, presumably) consider the presence of encryption as part of their is_suspicious() heuristic. Other people do have need for encryption, and by saying "I (currently) have nothing to hide", you are saying that you are fine with a high correlation between "uses encryption" and "is doing something suspicious". More than any other reason, we need to dilute that correlation until all data looks similar to remove the possibility of this kind of categorization.
2) https://www.philzimmermann.com/EN/essays/WhyIWrotePGP.html
As Zimmermann said, we need to socially normalize the use of envelopes instead of the postcards that are currently used. Without that social expectation, it will be possible to legislate against the use of encryption in the future.
3) It lets us (in the very long term) simply retire 80/tcp
...and plain HTTP servers in general. Sure, this is a minor benefit, but it would still be nice.