Basically it was a "belt & braces" approach in case the person hadn't been baptised previously or there was doubt about whether the original baptism was valid.
If it was a baby the original baptism may have been an emergency one at home because the child was frail or ill and not expected to survive. This may have been carried out by a member of the family, a midwife or doctor, or any Christian who was there and knew what to do. Should the child recover it may have been taken to church later for the rest of the baptism ceremony, with godparents, and other extras it didn't get first time. (The doctor baptised several babies in a rural parish in Lancashire which I looked at.)
In the case of a conditional baptism of an adult, s/he may have been:
converting to a different denomination;
joining a church and unsure whether they had been baptised as an infant;
about to marry in church and wishing to be baptised into the church beforehand, perhaps not knowing if they had been baptised when a baby. (Meghan Markle was baptised in C. of E. prior to her marriage to P. Harry.)
Conditional baptism in Catholic Church - explanation for family historians.
www.ottawavalleyirish.com/2010/09/conditional-baptism.html