Factually wrong those filler quests never give anything of utility.
Except they do provide Utility.
In BDO Quest exist Primarily to earn Contribution Points, which are used to Purchase housing/workshops and unlock Nodes across the Map, you will then be using this to set up workers and storage. This plays an important role as it helps to yield passive income in the long Run on top of whatever you make from mobs. There are also Adventure log books that are another form of Side quest in BDO that reward various Permanent Stat Boost and Storage.
FFXIV they exist Primarily as an alternative means of leveling, You can swap back and forth between this method and Dungeons. Same in WoW I believe.
And all In that, they exist mainly to flesh out the game world and provide lore/a sense of worth to what you are doing. This is all important in a Role Playing Game.
If you think questing Exist Just to artificially inflate game time then what exactly is grinding the same hand few of easy mobs in a circle in NGS?
Right Artificially inflating game time because there is next to nothing else in the game.