These are simply as they are called,
a program of the order of service for the marriage ceremony.
Whether you are being married in a field or in a church
these can add a nice touch to the ceremony,
and make a lovely keepsake.
Many churches have readings and hymns
that require the guest to have the words for participation.
This is also an easy way for everyone to know what's happening next.
They can be as simple as an A4 piece of paper folded in half,
to a ornate decorated program.