If I were to pick a section of any assessment submission that I struggle to award marks it would have to be either the introduction or the conclusion. Let's clear up some of the misunderstandings.
Whether it's an introduction or a conclusion, they are pivotally important in ensuring your assignment aligns within typical structures for essays but equally, this is applicable to other assessment formats too.
Purpose
The purpose of an introduction is to provide an overview of the key ideas or themes that will be explored within your submission. Many students try and complicate this process but a tried and tested method that has stood the test of time for me is the following structure. "This essay/submission will discuss ... (Important to utilise the key task words listed for the submission explore/discuss/critically appraise etc). The main reason behind an introduction is it provides the reader with a signpost of the key ideas and topics that you will explore and gives them a glimpse into your overall assessment and helps prepare them for the content to follow. This is often guided by the assessment task and illustrates the key parts of the rubric or task.
Similarly, if you were to consider a conclusion, it is for the same reason except it provides an overview of the key ideas or themes that have been explored throughout your submission. A good conclusion should recap or remind the reader of the key points discussed throughout this essay and this is often guided by the assessment task and you want to reiterate to your marker or reader that you have hit the key parts of the rubric.
Pitfalls
Typically, within an introduction and conclusion you don't introduce any new information or content, rather you are presenting an overall impression/summary of the content. Often I find that students begin their assessments with quotes or paraphrasing a theme for their essay but in most cases I do not believe it adds value to their submission. My general advice would be to summarise the key points and save concepts, explanations or literature until you are in the body of your assessment. However, I concede on the odd occasion a quote may work but this is the exception rather than the norm in my experience.
Think of the last time you had a good meal, do you really remember the main you were served? Or do you remember the entree and the dessert? This is not dissimilar to an essay.
