As far as I know, the NBA Divisions are subparts from the Western and Eastern conferences. I understand the impact of climbing the conference rankings as the top 8 teams in each conference receive a playoff spot. However, what is the purpose of Divisions aside from teams in their respective divisions play more frequently with each other?
Prior to 2016, a team's rankings within their division had an influence on how they are seeded.
In 2005, when the third division for each conference were first introduced, division winners were guaranteed the top three seeds, given the 1st to 3rd seeds according to their season record but irregardless of their record against any of the other 5 teams in the conference bracket.
This presented a problem in 2006, where the Dallas Mavericks were seeded 4th despite having had the 2nd best record in the West, behind the top seeded San Antonio Spurs, who happened to be their division rival.
In today's NBA, they would certainly get the 2nd seed but because they finished behind the Spurs in their division, the best they could get was the 4th seed.
They changed this system the following year in 2007 where the top two teams of the conference get 1st and 2nd seeds while the division winners get 3rd and 4th seeds, if applicable.
The system where division standings affected playoff seedings were then ultimately abandoned by the 2016 playoffs, who reverted back to seeding each conference according to conference standings.
The division splits are still there, however, now mostly just to help keep track of which teams play more games against each other more times over the course of the season.