Off the top of my head, some reasons why people would to marry despite intending not to have children:
residence permits
taxes (a pretty big deal in some countries)
warm fuzzy feeling about cementing a very-long-term relationship in the culturally approved way, signalling commitment
doing the culturally expected thing in a very-long-term relationship and not wanting to advertise their child-free status
Off the top of my head, some reasons why people would to marry despite intending not to have children:
residence permits
taxes (a pretty big deal in some countries)
warm fuzzy feeling about cementing a very-long-term relationship in the culturally approved way, signalling commitment
doing the culturally expected thing in a very-long-term relationship and not wanting to advertise their child-free status