Why do NBA Western Conference teams play against Eastern conference during season

The NBA has always had inter-conference play, as do most sports leagues other than Major League Baseball. Early in the NBA's existence, there were relatively few teams, so inter-division play was a must; for example, in 1960-1961 there were eight teams, playing 79 games, 13 intra-division and 10 inter-division per team.

Conferences were adopted in 1970-1971, and teams continued to play every opponent. Further, after the ABA-NBA Merger, when the number of teams started to approach modern levels (22 teams in that year, allowing for an 84 game season with each team playing each other 4 times). The NBA was somewhat unique in that schedule, having no preference for division or conference opponents, in fact.

That changed in 1979-1980, when teams began to play more against their own conference, 6 against each intraconference team and 2 against each interconference team. It has continued to ebb and flow over the years as the structure of the divisions and size of the league has changed (after all, the math has to work out to around 80-85 games).