I have understood that prompt baptism, particularly of sickly infants, was intended to ensure that in the event of death the child was assured of going to heaven (many denominations believed that baptism was necessary to be with God after death) rather than join the devil in hell.
As "Nick" was a name given to the devil this lead to the word "nickname".
As Frances said, later the given names may have changed to something the family felt were more suitable than those hastily used at the outset.
Cheers
Peter
Melbourne