There may have been many reasons, but we do know they were very adept at sailing very long distances. They could identify the direction of land a very long way away (based on cloud formations, wave pattern, sea birds, fish and I am sure other signs).

I have not read any literature about the motivations, but they may have gone a long distance and turned back if they found no land. They were probably explorers with the same human motivations as European explorers, the desire to find out what is out there whatever the cost, and we remember the ones that succeeded, such as Kupe

https://en.wikipedia.org/wiki/Kupe