If conducting any monetary transcations through your website then having an SSL Security Certificate is considered mandatory. This is both for your protection, your customer's protection, and compliance with varying compliance regulations.
Non-sales parts of your website are open to your discretion whether they need SSL security or not. You may want to secure sections of your site that involve user/customer logins and/or anything considered confidential or private.
Some sites use Self-Signed Certificates which is an option for non-compliance bound parts of websites. These are free to create and maintain on your own, but require the user accessing that part of the site to accept and acknowledge the Security Certificate was issued by an unknown authority (aka YOU).