Yes, this is purely for cosmetic reasons. Premier Balls have the same catch-rate as a standard Poké Ball, though people tend to use them for shinies because they like how they look and/or want to add a sense of self-worth to them. In other games, there are people who similarly use Luxury Balls only, color-code their Poké Balls, or use the Beast Ball on shinies for this exact reason.
For LGPE specifically, Premier Balls are the only unique Poké Balls in the game besides the usual Poké, Great, Ultra, and Master Ball obtainable, likely prompting people to use them as well.
