If the teacher marks the work, the teacher will have an insight into what students get and don't get about a topic and will be able to use this to plan the next lesson. They may even have an insight into how individuals are doing with given topics (but given the number of data points that weekly homework of several classes of 30ish, plus classwork, plus tests generate; they might only have a vague idea even when looking at their mark book)
When the teacher marks, the students often just look to see how many they got right and then ask their buddies what THEY got but not really look at "what do I need to work on to improve?"
If the student marks the work, the student will see patterns in what they are getting wrong and have more insight into how to make progress.
The student might not have the right level of reflection/meta-cognition to make those connections and just be going through the motions to generate a mark out of 10.
If the student marks another's work, the student will understand more about what was being assessed, which will be helpful next time they have similar questions.
If the work is more complicated than right/wrong, the nuances can be quite hard for students to make any sense out of their peer's project, and then they just focus on trivial aspects (such as neatness), or give inaccurate feedback based on their own level of understanding.
So, it's a balancing act.
I think as long as the teacher is grading big assessments themselves (so they have an idea how the individuals in the class are doing), it's ok for the students to assess the little bits and pieces like homeworks or pop quizzes. It might even be more helpful, educationally than just getting their work back with loads of red ink all over it.
Given how long it takes to grade maths homework, it's a much better use of the teacher's time to have them all graded at once over 15 minutes at the start of a lesson, so the teacher can use their non-contact time for lesson planning.
(I'm a science teacher who taught maths for one term and was shocked at the amount of time marking takes in that subject!)