Sinclair on some of the information in different list types

NATURAL FREQ LIST

• This frequency list [# 2 in Lextutor line-up] presents frequencies but in order of first occurrence. This can serve as a quick guide to new topics or major turnings in the text. If two word-forms have about the same frequency, but one occurs early in the text and the other does not occur until late, then this difference may alert the student to look for an interpretation.
  
• Again if ...there is very little technical vocabulary for some time and then a rush of it, that may be a clue to a high-level structural boundary in the text, perhaps the end of a general, layman's introduction to a technical subject.

FREQ ALPHA

The most-first list [# 4 in the Lextutor lineup] tells you which words recur in a text.

But in addition, if a text frequency list is set alongside a standard frequency list (say, from Brown corpus, available at Freq/lists_download) then this will serve as an informal keyword analysis. It will reveal the words that are far more frequent in this text than in the language at large. (It is good preparation for Keyword analysis to work with data manually/visually first.)