It all depends on the student. (Note that my children did not choose to repeat, so I’m not defending our own choices.)
Many students choose to repeat 9th grade for athletic reasons. They (or their parents) want another year of growth before playing contact, team sports. If a league permits PGs, younger boys may never get to be leading athletes.
Of the remainder, quite a few students choose to repeat a grade, particularly if they are changing from a public school to a private school. As private school curricula are frequently a year ahead of public school curricula, they are not repeating material. Many elementary and private schools stress grammar to a greater degree than public schools.
Some of the students who choose to repeat a grade have been allowed to skip grades in previous schools. They are seeking to join students closer to their own age. And some students want to experience a full four years in the same high school. It can be very challenging to start in 10th or 11th grade, especially if a student wants to gain leadership positions in extracurricular activities.
I haven’t seen a disadvantage in terms of college admissions for students who repeat. However, I’ll also say that I haven’t seen an advantage for those students either, apart from the students who are recruited for athletics. And I would worry about a student being significantly older than his peers.