If a cohort is synchronised to a course, it means that if I add users to the cohort, they are added automatically to the course it is synced with. Similarly, if I remove a user from a cohort, they are automatically removed from the course or courses that the cohort is synced with.
Say I have a cohort called First Year Maths 2018 that contains 20 students. I sync that cohort with the course pages that First Year Maths students need access to: "Basic Calculus", "Basic Trigonometry" and "Basic Algebra". This gives those 20 students access to those course pages.
However, 1 student drops out and switches to First Year Science instead, and 2 latecomers join the course. If I remove the drop out student from the First Year Maths 2018 cohort, she no longer has access to those 3 course pages. If I add the 2 latecomers to First Year Maths 2018, they immediately have access to "Basic Calculus", "Basic Trigonometry" and "Basic Algebra".
If I add the cohort in the way you describe above, by just adding the cohort members to my course pages rather than syncing them, the same 20 students will have access to the 3 course pages. The latecomers will not haven been added, and the student who switched to Science will still have access. I will have to go in to each course page individually and remove the dropout, and add the 2 remaining students. So in answer to Th Malta's question, this method doesn't synchronise the cohorts - that's not to say it isn't a valid way to enroll your students of course!