watasi to tomodati ha sorezore hon to zassi wo kaimasita.

Japanese lets you attach は to an entire noun phrase—even if it contains multiple nouns—so 私と友達は marks “my friend and I” as the single topic. If you marked each noun separately (e.g. 私は… 友達は…), you’d create two distinct topical statements, which changes the structure:
• 私は本を買いました。
• 友達は雑誌を買いました。
Here you’d be making two sentences or clauses. Using 私と友達は keeps them together as one topic sharing the same verb.

You can express “each” with ずつ, but it typically attaches to counters or numeral phrases (e.g. 一冊ずつ “one copy each”). A natural alternative would be:
• 私と友達は本と雑誌を一冊ずつ買いました。
This explicitly says “we each bought one book and one magazine.”
それぞれ is more flexible—an adverb—so it works without changing other parts of speech.

Japanese word order is fairly flexible thanks to particles. For example, you could say:
• 私と友達は本と雑誌をそれぞれ買いました。
• 本と雑誌をそれぞれ、私と友達は買いました。
All of these are grammatical, though subtle nuances in emphasis may shift. The most neutral order is topic → adverb → object → verb, as in the original.

Turning それぞれ into the adjective それぞれの makes it modify 本と雑誌 (“each person’s book and magazine”), shifting the nuance:
• それぞれの本と雑誌を買いました。
This implies “(We) bought each one’s own book and magazine,” focusing on possession rather than distribution. It’s less common if your goal is simply “we each bought a book and a magazine.” For that, the adverbial それぞれ is clearer.