Correction: A lot of traits are adaptive—but definitely not all. If you want an example of a trait that isn’t adaptive, look no further than your own belly button. A belly button is a scar left from the umbilical cord connection that sustained development. The umbilical cord? Definitely an adaptation. Its scar? Not so much.
The belly button is an example of a non-adaptive byproduct, but there are other kids of non-adaptive traits, too. For example, tetrapods, such as humans, typically have five digits—why five? There is probably no good reason—it just happened that way. Fingers and toes are of course adaptive, but the specific number of them? Not so much.