Working alone builds students' own individual skills, knowledge, and confidence and permits a more accurate evaluation of each student's weaknesses and achievements. Students may NOT ignore the rules of the course because they don't like them, because think they learn more by working with others, or because they get frustrated when they can't figure out the answer. And they can't avoid responsibility by saying they never knew there was a rule against working together - ignorance is not a defense. Or they may permit students to work together on the early stages of a project or lab, but require the final write-up to be in the student's own words. It may be okay for students to discuss general concepts of a homework task, but be required to calculate the final answer on their own.
Unauthorized collaboration misrepresents joint work as the work of an individual.
Unauthorized collaboration gives those who break the rules an unjust advantage and results in unfair competition. Those who always work with others may be unaware of gaps in their own knowledge and skills, and do not learn all they can or should from assignments. Partners may discuss the concepts involved in the experiment and the lab report format. Preparing graphs and writing up the report are important elements of the learning process, which each student was required to do alone. Even if neither copied, and they worked together only on the graphs, they still broke the rule.