Sure, because senior government officials sometimes do things outside of their official capacity. If a senior government official's communications with their therapist or their lawyer e.g. are compromised, this could leave them vulnerable to blackmail. That's bad for everyone.

Re: other post about use of Signal, I'm not commenting on that post and/or that issue. I'm not expressing an opinion either way on whether it's relevant to this issue. I just want to point out that FedRAMP exists, and Signal isn't FedRAMP certified. If you don't think that is important (and a lot of reasonable people don't), you do you.