Building software is like baking. Sure you can bake a cake from scratch, or you could pick up a box mix from the store and have a cake in half the time.
The build tools are the box mix: it puts together basic parts of the recipe and has them ready to go for you. Combine this mix with the rest of the recipe (like eggs or in our case, your code) and you have a finished product.
So yes, you can work without them but it will cost you lots of time. Also, when you bake a lot of cakes, you likely want them to be consistent. Same for apps. The box mix comes in handy here because it guarantees that part of the recipe will always be the same.
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