It is also relevant to the design of disk memory systems, specifically relating to what happens when they are unexpectedly shut down.
As a relevant backup example, consider a website with a database such as the online encyclopedia Wikipedia, which needs to be operational around the clock, but also must be backed up with regularity to protect against disaster.
For example, some structures are indexes which permit the database subsystem to quickly find search results.
Assume Wikipedia's database is a huge file, which has an important index located 20% of the way through, and saves article data at the 75% mark.
Consider a scenario where an editor comes and creates a new article at the same time a backup is being performed, which is being made as a simple "file copy" which copies from the beginning to the end of the large file(s) and doesn't consider data consistency - and at the time of the article edit, it is 50% complete.
When properly designed, they ensure that data will not be unrecoverably corrupted if the power is lost.
A disk caching subsystem that ensures point-in-time consistency guarantees that in the event of an unexpected shutdown, the four elements would be written one of only five possible ways: completely (1-2-3-4), partially (1, 1-2, 1-2-3), or not at all.
High-end hardware disk controllers of the type found in servers include a small battery back-up unit on their cache memory so that they may offer the performance gains of write caching while mitigating the risk of unintended shutdowns.
The controller essentially lies to the operating system and reports that the writes have been completed in order (a lie that improves performance at the expense of data corruption if power is lost), and the battery backup hedges against the risk of data corruption by giving the controller a way to silently fix any and all damage that could occur as a result.
Suppose a money transfer requires two operations: writing a debit in one place, and a credit in another.