The point of it...
An index file knows the position of every single word in your corpus and it is possible therefore to ask it to supply specific data. For example, the lengths of each sentence or each text in the corpus (in words), or the position of each occurrence of a given word.
How to do it
With an index open, choose File | Export index data,
then complete the form with what you need.
Here we have chosen to export the details about the word SHOESTRING in a given index, and to get to see all the sentence lengths (of all sentences in the corpus, not just the ones containing that word).
A fragment of the results are shown here:
At the top there are word-lengths of some of the 480 text files, the last of which was 6551 words long; then we see the details of 5 cases of the word SHOESTRING in the corpus, which appeared twice in text AJ0.txt, once in J3W.txt etc.; finally we get the word-lengths of all the sentences in the corpus : the first one only 4 words long.
This process will be quite slow if you request a lot of data. If you don't check the sentence lengths you will still get text lengths; it wil be quicker if you leave the word details space empty.