Most single-player open world games could only dream of having an online counterpart equivalent to GTA Online's success. Having both GTA 5 and GTA Online as a package deal is a steal for how much content there is. The player can devote hundreds to thousands of hours of their life to these games and still have plenty to discover.
Either game by itself is a masterpiece. Hence, having these two titles together sets the bar for other open world games to try and replicate. There are plenty of good single-player and multiplayer games, but not often packaged together.
This reason helps some players who might prefer either GTA 5 or GTA Online, as they're not forced to play the other game. Both games are largely played on the same open world, but that's not a major detriment.