Why is there an AFC and NFC in american football, what is the difference?

For around 60 years there was around 12-16 teams in the NFL. Then a rival league, AFL, was born in 1960. The AFL competed well with getting star players, unlike other leagues that tried and failed. After about 6 years or so competing, the NFL decided to absorb the AFL teams and their was an agreement that they would do so in 1970. They had 2-3 Superbowls before this merger against the NFL and AFL. They agreed in 1970 that there should be 2 evenly sized conferences between the 26 total teams, therefore 3 teams from the NFL (NFC) moved to the AFL (AFC), Baltimore Colts, Cleveland Browns, and Pittsburgh Steelers. Creating 2 conferences with 13 teams each. They kept rivalries by keeping 2 different conferences.