SSL is not just about encryption. SSL certificates also provide the end-user with an assurance that you (the web site operator) really are who you say you are.
In simple terms, the certificate issuing authority is vouching for you and your organizational and contact information presented in the certificate.
The main differences in certificates are (1) the reputation of the issuing authority, and (2) the level of diligence the issuing authority will perform to issue the certificate.
Regarding (1), anyone can generate a secure certificate. Just download OpenSSL and Google for instructions. But certificates are useless for public use unless they're issued by an source that is well known and trusted.
To expand on (2), cheap certificates usually provide very basic validation; they just verify that you have administrative access to the domain (by emailing you at the domain, for example). Expensive certificates generally require a much higher level of verification by the issuing authority (they might actually call you, require you to submit organizational documentation, etc.) and therefore can offer a higher level of trust to the end-user.
In real world use, most end users aren't educated enough to care about the level of diligence performed and the quality of the certificate; all they need is to see the little padlock icon and not receive error messages. But if your business can afford it, a premium certificate and the added level of trust it provides is a small price to pay to assure the small number of users who *can* tell the difference.
