Welcome back to the forums!
I'm new here but my mindset is simple - I feed messy beefheart multiple times a day and it makes my water nasty dirty. I can smell it. Meat spoils and as it breaks apart into cloudy nastiness and I know it gets mixed in with the water so I feel compelled to change it.
I also feel slime buildup on walls and tubes and wood, so when I wipe everything down, it makes the water dirtier as well.
Not to mention sponges pick up bits of food and poop all day, so I gotta give them a rinse as well.
Combined all in all the water gets dirty - fast.
But the number one reason I do my water change is because I love my fishies and want to give them the best.