It's mostly a matter of efficiency. Yes, Log Luffy is pretty much a straight upgrade to G3, however there is certain content that is straight up better or easier to do with G3 because Bandai's balancing of future events by adding things like Perfect shields on enemies. Similarly, Rayleigh does less damage than G3 as a whole (though it's still huge) but is also INT cerebral, which is just straight up easier and better for certain content, like Boa raid. Bandai usually throws interesting twists to content to handicap certain units, such as combo limiters in battles for Rayleigh.
The nice thing about the game is that it isn't one size fits all. Eventually, you'll want a number of different high level units for different utility.
In terms of raw damage output on a rainbow team, the only character on Global that beats G3 is Log Luffy, I believe. But raw damage doesn't tell the whole story. That said, G3 is a fantastic starter for early and late game, and will carry you through all the content on global currently.
