Why should I use SSL?
Most people starting a webshop knows they will need a website with goods, services, information and a way to order and pay for the goods. However, most don't think about that that they need to make the customer feel secure and protect the communication between the customer and the webshop.
When a customer feels insecure they will most likely not go through with the transaction, so all measures that are being implemented to make the customer feel safe will increase the number of completed transactions. These measures can be everything from a physical address and a phone number to an SSL-certificate that proves the company's identity and encrypts communication between the customer and the company.
Apart from that you also protect yourself and your customers from the information being shared between you can be abused, and that someone makes a false website that looks like yours.
Why do I need an SSL-certificate for my company's webmail/intranet?
Most companies want a level of security on their systems where people not related to the company can read internal e-mails, documents, internal price lists, marketing strategies, etc. When this information is being accessed from external machines over the internet, fx. through webmail, it's important to know that the communication between the client and the server is protected.
Intranet servers like Sharepoint and webmail clients like Outlook Web Access are most often not protected by a self-signed SSL-certificate. A self-signed certificate will cause issues for external machines that tries to access the site by showing errors and warnings to the user, and for mobile clients it's often not possible at all without manually changing some settings on the device.
With an SSL-certificate you protect the data with encryption, including the user's login and password when they access the site. Your mobile devices will also be supported (depending on the selected certificate) and you will avoid error messages in the browser. All in all you get better protection and easier access.
The choice of certificate depends a lot on which server that is to use the certificate, how many domains are to run on the server, whether mobile devices needs to access the site, etc., but it's seldom profitable to use a self-signed certificate compared to a cheap one like AlphaSSL or GlobalSign Domain SSL.