Writing is about presenting ideas, and it is important to tell the reader how the ideas relate to each other, which is done by using linking elements.
There may be more than one idea in a sentence, so linking would be used there:
I love chocolate BECAUSE it is very tasty.
Here BECAUSE links the two elements of the sentence, and tells the reader why I love chocolate.
I went to London yesterday to see my sister. THEN I went to Paris to see my brother.
Here the word THEN links the two sentences, by telling the reader which event happened first.
If I write three or four paragraphs which are part of the same theme, then I could use SEQUENTIAL LINKING, such as FIRSTLY, SECONDLY, ADDITIONALLY and FINALLY.
If I write some paragraphs where the ideas are in opposition to each other, I could use the following linking:
ON THE ONE HAND, ON THE OTHER HAND, and ON BALANCE.
The student doesn't need to learn a long list of different linking words, but it is important to understand why linking is used, where it needs to be put, and to have an appropriate linking word for the situations that could come up.