As an addition to the other answers:
There may also be legal reasons. For example, in Germany all business communication (email, letter, fax) sent by a company must contain certain information about the company, such as its name, address, company registration number, managing board etc (see e.g. Handelsgesetzbuch
§ 37a ).
Violating these rules can cost the company a heavy fine, so most companies put this information into their standard email signature. Strictly speaking, the information is only necessary in mails to recipients that you have (or intend to have) a contractual relationship with - so it's no necessary in internal mail, or if you are merely requesting information. However, since this distinction is complicated, most just always include the information.
I believe some other countries in the European Union have similar rules.