Futuralis has a good take on why the (admittedly expensive) upgrade is worth it.
Let me add two more: the geometry of islands and as insurance in certain matchups.
Islands tend to be squat and narrow; long rectangles instead of squares.
This means once you have won water and are bombarding the shore (really the only time you should be making CGs anyway; unless you’re Spanish?) the extra range can help you hit all corners of an enemy’s base. 15 range is basically a Treb, whereas 13 is a BBC with SE; the extra two range can matter a lot on land and it’s the same on water.
Keep in mind you only need to tag the corner of a building to bombard it down.
The other thing is it might be required in certain matchups.
Crenellations Castles, Eupseong Keeps, or Artillery BBTs/BBCs can range CGs but can’t hit ECGs because of the extra range.
It’s particularly fun as Turks to get the Elite upgrade (cheaper and faster!) and Artillery and nuke stuff from across the Pacific Ocean with 17 range, 11.