The broad based answer Nick...is multi faceted. And was certainly further enhanced viz the age of enlightenment imo.
First and foremost for the expeditionary leaders: personal fame, peer and national-international recognition of skill or field of expertise, fortune and advancement; as had been identified by the examples of their earlier expeditionary predecessors.
Secondly and certainly, ala the post Cook and Darwin expeditions, for the purpose of exploring unknown regions and climates to seek out opportunities for scientific experimentation or discovery.
Thirdly: Nationalistic identification and expansionism through acquisitions of territories and possible development of acquired resources. Which might then further trade and markets, establishment of routes (no matter how seemingly far fetched in the vicinity of those latitudes) and economic power.
Those have been the motivations that I have identified that would indicate a norm.