Everything about biology is a random effect. Even a mutation that's selected for wasn't planned; it just happened by chance. Like if you're an aquatic species maybe you'll end up being a strong swimmer over generations, but the water doesn't pressure you towards that on its own. You have to coincidentally develop flukes that make you a stronger swimmer before those traits can be selected for.

Sometimes traits that get passed down aren't beneficial at all because they don't make an impact on reproduction. Think of an animal that comes in many colors like a house cat or certain fish species. In such cases it's clear that the color of the animal doesn't have any bearing on its ability to reproduce, so a variety of colors are passed down for no particular reason.