It is true that coherence means the ideas in a paragraph are logically connected.
Let us take our perspective in essay-writing for example. In an essay, there is already a topic given, all you have to do with the successive paragraphs you are writing is to write things related to the topic given. Every paragraph in the essay must be a subtopic to the overall topic therefore having another topic in each paragraph will make your essay incoherent and unparalleled in ideas perspective