I play all the routes because I believe that a game isn't complete without that because all routes contribute some way toward the overall work and narrative of the game. Even if I prefer a certain route, the other routes would only enhance it because it would highlight the individuality of every love interest (including of the routes I like more), different facets of the MC, and the specific themes of each route through contrast. Like is this characterisation/story element something unique to this character that I should view as something important to them in particular, or is it shared amongst other characters (and what does this say about the themes)? It helps to understand each individual character and route better by defining each theme, etc, more clearly through what is shared and what is unique.
Not to mention how each route enhances and contributes to the overall narrative and themes (at least it should), with many games not revealing the full story through only individual routes but instead the entire game as a whole, requiring all the routes to be seen. At least the subtle nuances will only be understood after seeing everything, even if routes do (and should) make sense when taken by themselves as standalone stories. I am also a completionist and want to see everything a work has to offer, and I feel like it's a way for me to respect the creators and writers to read everything they crafted - if they wrote it in, then I would consider it important to the work in some way. Even if it's small, I want to know every bit of information. Nothing is filler to me.