Learn Italian: Questo zaino è sia comodo sia leggero per il viaggio. - This backpack is both comfortable and light for the trip.

It translates to "both ... and", emphasizing that the backpack has two qualities at once. You will also very often hear Italians mix the conjunctions and say sia comodo che leggero, which means the exact same thing.

Could I just say comodo e leggero instead?

Yes, absolutely! Saying comodo e leggero simply means "comfortable and light." Using the correlative conjunction sia ... sia adds extra emphasis, just like choosing to say "both comfortable and light" in English instead of just using "and".

Since the word for backpack is lo zaino, why doesn't questo change its spelling here?

Unlike the definite article or the word for "that" (which becomes quello zaino), the word questo (this) is completely regular. It doesn't have a special form for words starting with "z" or "s + consonant". You just use the standard masculine singular questo.

Why is it per il viaggio and not just per viaggio?

In Italian, nouns generally need a definite article unless they are part of a set phrase. Because this refers to a specific trip the person is taking, the article il is required. You would only drop the article in set expressions like essere in viaggio (to be traveling).