SQL - CoolTShirts marketing attribution (code discussion)

hello everyone,
I really need to discuss my question about the project code. I could not understand why we should use join in this project and I have tested my code without join and it returned the same result for every question. For example to calculate the first touches attributed to different campaigns. Below is my code. I would appreciate if someone can discuss with me, thanks!

with t as (
select user_id, min(timestamp) as first_touch, utm_source, utm_campaign
from page_visits
group by 1)
select t.utm_source, utm_campaign, count(*)
from t
group by 1, 2
order by 3 desc;