Editing work is a normal process. Every book or journal article you read will have been edited several times. No-one can write perfectly, so you need to check that your work is clear and without errors. Lecturers often point out that work contains spelling and grammatical errors, and this can lead to lower marks. Many assignments will have a criterion about this, so you may see a criterion worded something like one of these:
You will often get around 10% of the mark for this, so that could be the different between grades, or even passing or failing, so this stage is extremely important in writing your assignment.
Some Schools in the university will have provided you with guidance for assignments, so checking that you are writing in the correct font, size, using the correct line spacings is very important.