Does the student portfolio help learners and teachers improve learning and teaching?

Yes.
A child will grow and develop over the course of a school year.
Having a portfolio is a wonderful idea that allows both the teacher and the child to see the progress that has been made.
For the child, it provides a sense of accomplishment, pride, and is very encouraging.
For the teacher, it is a learning tool.
The teacher is able to assess the growth of the student in many different content areas to determine their strengths and weaknesses.
From here, the teacher decides if the child is ready to advance or if a refresher lesson is needed.
Portfolios should be looked at as an untapped resource where the teacher can also assess their personal strengths and weaknesses, based on the content absorbed by the child.
It is also great for showing the parents during Parent Teacher Conferences because it gives the parents a visual.

Yes, it helps learners to know their strength and weaknesses and helps the teachers to assess and evaluate the learner's capability.

Of course, the traditional method of teaching has expired a long time ago.

In my opinion , observation and documenting children progress helps a educator to view and assess the environment of the classroom to meet the children needs/strength.

Yes, it helps teachers to understand his student's calibre and accordingly a teacher can design his teaching material and change his style with those students.