A long time ago, in the 90s, there was no streaming. Networks needed to fill their daytime slots, so they put "old" episodes of a series there.

A series needed about 100 episodes to air 5-10 episodes per week without airing a duplicate episode every other month.

Also television exploded in the 1980s with satellite TV and cable. The networks needed more content.

Baywatch was briefly the biggest series on earth.