Logging and telemetry are completely separate use cases. For example to do some kind of battery use accounting you need some record of when exactly which app was active.

And no, it's not widely known or documented - there is no good description of what telemetry exists or contains on iOS that I know of.

don’t you need to enable analytics?

first compressed and then encrypted. A good encryption is indistinguishable from random data.

That's why it's compressed before encryption?

Yeah, because encrypted data should be incompressible, as it should be indistinguishable from random data, which is also incompressible.

Reality is a little different of course, and compression can cause problems for encryption because compressed data tends to be highly predictable (especially things like compression headers and compression dictionaries). This allows for potential “known/chosen plaintext” attacks on the encryption.

Some classic examples of this type of attack are breaking Enigma (known plaintext, no compression) by assuming the content of some messages[0] and the more recent CRIME[1] attacks against TLS using compression to help produce a chosen plaintext.

The simple solution in these scenarios is to avoid using compression completely.

[0] https://www.quora.com/Did-the-inclusion-of-Heil-Hitler-at-th... [1] https://en.m.wikipedia.org/wiki/CRIME