The ILM defines metadata as and the need for it succinctly:
Generally, metadata is information that describes how, when, and by whom a particular item or set of electronic information was collected, created, accessed, modified, and formatted. The questions above arise in the context of examinations of taxpayers that keep their business records electronically with metadata automatically created as an integral part of the records. In many instances, the Service's examinations would be advanced by accessing metadata that identifies the original date a transaction was entered in the electronic records, the dates of any changes to the entries, and the username of the person who made the entries. The value inherent in an examiner's ability to obtain the date and source of recorded entries is self-evident; the information tends to support or undermine the credibility of the entries in the business records.See also for types of metadata, the Wikipedia entry on Metadata, here.
Of course, that the IRS can summons metadata does not mean that it will summons metadata. I dare say that, in most civil audits where there is no suspicion of fraud, metadata would not be useful and hence, perhaps after some flirtation with it, the IRS will be more discriminating about the cases in which it requests metadata.
I suspect that in criminal investigations, the request for and mining of metadata may be more common. I know that, in some (perhaps all) of the major tax shelter criminal prosecutions in NYC, the accounting and law firms receiving grand jury subpoenas produced the electronic files. What I don't know is whether the prosecutors attempted any type of mining of the metadata and, if so, whether it was useful. But, the expectation is that with the issuance of the ILM, the IRS is indicating a willing to hone its ability to make use of such metadata.