Honestly and maybe not all that surprising I think the writers and probably also people knew or at least had an idea that everything did not come into existences at the same time.
Just like we today, ask the question "What came first the chicken or the egg?" I think they would have wondered the same, people weren't stupid back then. But they had no real way of answering such question. Today we can answer this through evolution, but to them it have probably been a mystery of how that could be possible, therefore God must have created everything in their complete form, which makes perfect sense if you are a person during that time period and the word evolution doesn't even exist.
But you can take this further assuming we lived at that time... you might wonder the following: "For the Goat to live it need to eat grass, now the grass doesn't eat anything, but grow on soil and to grow it needs water." So logically it would make sense, if the soil or water came first, then the grass and then the goat. Which is exactly what we read in Genesis, first the water, then the dry land, then vegetation, then light (Sun / moon) and then the animals.
Maybe they thought that if God made it like that, then they had a rather logical explanation of how things were made. At least to me that sound plausible for them to draw such conclusions. So by splitting the creation into days it helps them set the order in which God created things, rather than just have one verse saying, "God created everything as he saw fit.... Adam and Eve ate a fruit..."