Bonjour Tatiana !
Here there's a nuance of meaning between une demi-baguette and la moitié de [la] baguette.
Une demi-baguette means "one half of baguette" or "half a baguette": here it's a general characteristic of the object, as opposed to a full baguette.
La moitié de [la] baguette means "the half of baguette" or "half the baguette", implying that we're talking about a specific baguette here, of which you'd want half.
The sentence here could be heard in a boulangerie, where you would ask for half a baguette, hence une demi-baguette.
I hope that's helpful!
À bientôt !
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