The conclusion is an important element in writing any document in which there is a discussion of views, opinions, recommendations and research findings. However not all documents require a conclusion. For example, a Strategic or Operational Plan would not require a conclusion but a report on research of customer opinion probably would.
The conclusion is the final opportunity for the author to reiterate and emphasize selected key points previously presented in the document. However the reiteration of key points should be brief. There is no need to add further explanation to key points in the conclusion. If more explaining is needed consider re-editing the document and putting the further explanation earlier in the document.
It is important not to introduce new arguments, ideas or topics in the conclusion. It is easy to off at a tangent with one's thoughts, especially when there is writing fatigue and a desperation to finish the document well.
It is permissible, for some types of documents, for the author to make a brief statement of their own opinion in their final concluding remarks. However, be wary of writing any in depth personal opinion in the conclusion. Generally the reader of the document will be able to discern the author's stance on the topic by reading the rest of the document.
The length of the conclusion is a difficult question that few experts will put into figures. Most people agree that the conclusion should be brief but there is a risk that the author will appear to have made insufficient effort if it is just 2-3 sentences. In a document of 1000 words, the length of the conclusion might be 60-80 words and this might increase to 100 words in a document of 2000 words. The length of the conclusion therefore does not expand at a linear rate with longer documents.