how do things that aren't created serve a purpose?

Nothing is created, nothing has a purpose

We just think it has - see Douglas Adams's "puddle" analogy...

"Imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!' This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it's still frantically hanging on to the notion that everything's going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for. "

They don't. Purpose is something that only minds are concerned with. Our minds are comforted when everything is ordered and purposeful, so we seek out order and purpose. There is nothing wrong with that, we just run the risk of beginning to believe that this purpose and order exist outside of our minds.

"the sun, sky, land, sea, trees all seem to function with some sort of purpose"

This isn't purpose any more than a puddle of water "purposely" and perfectly fills the whole it has gathered in. Things that cannot exist within the confines and limits of the natural world cannot exist, period. It has nothing to do with purpose and everything to do with possibility.

Plants that give off oxygen serve a purpose. Some even supply us with food.

I'm not sure all creations have a purpose. Well not a worthy purpose.

GOD’S purpose for the earth is really wonderful. Jehovah wants the earth to be filled with happy, healthy people. The Bible says that “Jehovah God planted a garden in Eden” and that he “made to grow . . . every tree that was pleasing to look at and good for food.” After God created the first man and woman, Adam and Eve, He put them in that lovely home and told them: “Be fruitful and become many, fill the earth and subdue it.” (Genesis 1:28; 2:8, 9, 15) So it was God’s purpose that humans have children, extend the boundaries of that garden home earth wide, and take care of the animals.

WHAT THE BIBLE TEACHES

God cares about you personally.—1 Peter 5:7.

God’s personal name is Jehovah.—Psalm 83:18.

Jehovah invites you to draw close to him.—James 4:8.

Jehovah is loving and merciful.—Exodus 34:6; 1 John 4:8, 16.

https://www.jw.org/en/publications/books/bible-tea...

For that matter, what purpose human serves except destroying mother nature. Cockroaches are the oldest surviving species of earth and they roam in this planet at the times of Dianasoures billion years ago.

Exactly, not everything has a purpose.

Specifically, nothing in the natural world has a predetermined purpose- flowers aren't there to help bees make honey, clouds aren't there to make flowers grow, people aren't here to worship a magical sky daddy.

You created this question and it serves no purpose, other than to make you look a bit stupid!

Teleological fallacy. Not everything serves a purpose.

Why do things need a purpose?

"...seem to function with some sort of purpose..."

"Seem" to who? You? Your personal feeling is not evidence of "purpose."

Peace.