Including email addresses on your guest registration card allows you to communicate immediately and directly with these potential members. It also allows you to get feedback on how your congregation is perceived. A simple tool for collecting this data is an online survey service like SurveyMonkey.
These kinds of online services also allow you to "filter" the results, allowing you, for example, to see results only from people with children; the Lesbian, Gay, Bisexual, and Transgender (LGBT) community; or those who came at the invitation of a friend or family member. If desired, you can supplement with a paper survey for people who do not give you an email address, and manually input their data into the online tool.