Some of the stuff included in email signatures is there because there is a legal obligation for it to be. Perhaps not all the legal waffle is required, but a certain amount of signature content is.
What a lot of people don't realise is that under the Companies Act 2006 Private Limited, Public Limited or Limited Liability companies (so most companies in effect), email correspondence to people outside your organisation must include your company name, registration number, place of registration and registered office address. This is the same as the requirements for hard copy letterheads and order forms in the Companies Act 1985.
Many of course do have this on hard copy correspondence but are often missing one or more or all of these elements when it comes to emails.
Of course this wouldn't apply to interdepartmental emails within the government, but turning signatures on and off all the time is a pain, and of course as others have said why print emails at all?
The only time I've ever printed emails is if they're required for a meeting, some form of disciplinary procedure or in response to a DPA SAR.