Cohesion refers to the unity of ideas in a paragraph. Every detail in a paragraph should support the main idea.
Having cohesion in a paragraph or the entire text is important.
Readers do not only understand sentences individually, but they also take the meaning of the group of sentences as a whole into consideration.
When a paragraph or an entire text lacks cohesion, readers may find it challenging to determine how the ideas in it are related to one another.
Each supporting sentence in a paragraph should support the main idea or the topic sentence of the paragraph.
When ideas in a paragraph support the main idea (cohesion), it will be easy for readers to understand the text or the connection between ideas.
If a paragraph lacks cohesion, readers would be confused and find it difficult to determine the main idea and the supporting ideas as well as the connection between the ideas.