Folio, as in its literal meanings also, means a sequence of number of words for the purposes of dividing a book into meaningful parts or just for reference.
Same goes for its use in accounting as well. Folio coloumn is used in Journals as well as in Ledgers. It usually contains the reference in forms of short codes of relevant accounts and journals with page numbers for easy linking between journals and ledgers and referencing.
In journals folio coloumn is used to mention the reference or “address” of ledger in which the journal entry has been posted thus giving an easy access and also easily understanding whether all the entries has been posted in the relevant accounts or not. If a particular journal entry does not have a cross-reference to the concerned ledger then it might mean that it has not been posted yet to the ledgers. To learn more about journals and their types please read What is Journal and how many types of journals are in accounting?
In ledgers, folio coloumn is used to provide a reference back to the journal from where the posting has been made so that if accountant is not sure about the posting then he can go back and check it from the journals as well as there he will have not only the amount but also the date, description and the other accounts being affected in the transaction.
Folio coloumn makes things much easier to control and also error free. Imagine that thousands of transactions are required to be posted to the ledgers in hundreds of account and now you want to trace back a particular posting then without the “address” of the journal from where this posting came you will be lost and will have no clue how the entry was initiated and who approved it and all the other aspects. So, its a simple, precise and beautiful way to promise many things related to bookkeeping.
Many books when giving examples and providing solutions to the questions make up journals and accounts in the ledgers with folio accounts but left them empty as most of the time author intends to discuss some other concept and thus concentrates more there instead of filling out extra coloumns which might make students more confuse. But where author thinks that students who should have reached at this page of the book must have acquired enough understanding then he might make up folio coloumns and fill them with appropriate information. There is no other reason to my mind for leaving folio coloumns empty in the books.