Many businesses have more-or-less standard email signature formats. Some go so far as to dictate font pitch, font selection, spacing, etc. Others only ensure that your name, title, and contact info is readily available.
Why should (or should not) an email signature standard be used? What benefit(s), if any, does it provide?
First and foremost this is done for branding and marketing.
Most companies have a logo and mandate its usage when corresponding in an official capacity. Why?
Because it is part of the company brand - making it recognizable.
Company email signatures are no different - they show the company brand and are a marketing tool.
Having a unified signature across the organization is the professional thing to do. Some companies are more aware than others in regards to fonts, pitch, color selection and such.
Other reasons (mentioned in other answers) are to ensure that people don't personalize their signatures - why would a company want to avoid that? Because they may not want to be associated with certain personal passions of certain employees (politics and religion for instance) and to ensure a certain professionalism (banning animated images or some inappropriate imagery/content).
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.
Standards can be used to establish a floor and/or a ceiling.
Specifying the exact graphic file to be used minimizes the space on the mail server, because it makes sure that the graphic file size is not humongous. It also limits the use of inappropriate graphics, or animated gifs (unless that is what they want you to use)
Specifying minimum content means that external and internal customers can contact you even when they can't click on you email address. This is great for emails that end up being printed, or heavily forwarded/replied emails.
Font and spacing rules make sure that the signature doesn't take up too much space, or is unreadable. Which can be important when many emails are read on phones and tablets.
The development of standards usually happens when there is a perceived issue that needs to be resolved, and the non-standardization of email signatures from a part of the company are deemed to be a part of the problem.
This is also to prevent people from using something inappropriate as part of their signature like religious or political quotes. Since emails from the company represent the company, they don't want personal stuff in it that the company may not want to be associated with.
