Within a company, you usually want to control the signatures that users are using when sending mail outside of the company. Most companies want to include at least a "disclaimer" text, others want to control the amount of contact details as well and still others want to control the entire look and feel of the signature. Outlook offers no direct means to do this since the Signature feature in Outlook is a client side feature and thus users can create and modify their own signature. You can lock down the access to the Signature feature by using Group Policies but that still doesn't take away the issue of creating/generating a standard signature in the first place and deploying it to the users.
This guide discusses the features that Exchange offers to create and manage signatures at server level. Also, additional methods are being discussed to manage it without an Exchange server. Finally, an overview of 3rd party solutions is provided for alternative solutions.