Outlines are different from timelines.
Outlines are the writer's guiding path, and can be super strict, or super vague. They're generally made at the start, or when the writer's become lost so they need to focus back onto the keypoint. The "Hero's Journey" is an outline.
Timelines on the other hand are a systematic approach of keeping track of when things happened and are adjusted. Every story has a timeline, but not every story has an outline.
Example Spirited Away: Family moves to a new town. Family gets lost, visits a theme park. Parents eat magical food and turn into pig. Little girl runs away from the pigs, not realizing they're her parents, and runs into a young boy who tells her to leave promptly.
Chihiro never ever meets Kohaku before her parents turn into pigs. Her parents have to turn into pigs for her to be scared to run around the magical city to even run into him.
People use them for various reasons, but namely as tools for hard facts to be settled. Another example is if you just read a story without taking some passage of time into consideration, then you need to assume it happens really fast. Greek Tragedies / Comedies, for example, never happen on a boring day of the year. Lots of people reading The Iliad don't realize right off the bat that it takes the "protagonists" years to sail to the battlefield, and it takes Odysseus ten years to get back home (part of the reason his son's grown up upon returning home, and why the suitors are so desperate to marry his wife for her dowry).
My point is that, as I said, every story has a timeline, whether you write one or not. A simple trip to the grocery store has a timeline:
- enter the store
- walk right
- look at produce
- skip the soda aisle
- go down the cereal aisle
- pick out candy cereal
- go to checkout
- pretend to be polite to the cashier
- pay in cash
- is short 10 cents
- cashier says "Don't worry"
- thank cashier
- hurry out of store
Now a few ways to spruce up the story is spend more time on one or more item. Explain why you skipped the soda aisle: don't need soda, don't drink soda, when did they quit soda, is freshly quitting something else which is stored by the soda (usually alcohol or breads). You don't just skip the soda aisle without a reason, and even if you as a writer don't say why, the readers will infer their own guesses and make up a backstory reason.
Which is another point: backstory.
Most stories have multiple characters that have different backstories, and timelines make it easier to marry their chronology together. Even if the reader never sees it, it's important for the writer to know when something happened so they can refer to it properly.
An example is from my own story, where Crow and Will knew each other when they were younger and met several times. It stands to reason no one is the same each time you meet them. The reason Crow's behavior changes between each meeting with Will as they grow up is due to Crow's homelife circumstances changing. Narrating from the start where Will doesn't recognize Crow to "Hey I know you, what happened?" to Crow either telling him, or Crow being forced to have flashbacks to unhappy points in his life (again, nothing happens on a boring day, for better or worse). It's important for me as the writer to know the chain of events that helped shape Crow's current day character.
No one starts off brooding or happy all the time. Look back to The Iliad. Someone has to die who's important to Achilles to really set him off to take part in the central battle and before that he's a brooding arrogant man - why - Because we know he's got parental issues (putting it lightly for Greek mythos).
If we just had Achilles as an arrogant brooding character with no substance, that turns people off to being invested in the character. There's a reason a lot of workshops are titled things like "how to make multi-dimensional characters!" Lots of it involves working out their past and putting that into the overall timeline. If you have Character A go to the supermarket to get candy cereal on the same day Character B went there to get meat, did they see each other, did they interact, did one see the other but wasn't noticed themselves, why are these important? Because later Character A tells Character B they went to the market to get something, and B has a choice to chime in with the truth or lie: I saw you, vs, I didn't see you. Both choices effect what kind of perception the reader has of the characters and their relationship. Maybe it's two friends post-fight avoiding each other, maybe it's kids avoiding their parents, maybe it's kids happy to see their parents, maybe it's two neighbors who both forgot to look for hedge clippers. Reasons are endless.