A very common scenario in the lifetime of any given document is that person (A) writes a draft, sends it to B,C,D for review, receives feedback from each of them (separately or together), A updates it and the cycle continues until you have the finalized version. And quite often the files are exchanged by email.
Sounds simple, right? Well if you say yes - you have not actually dealt with the very real problem of version control (or change control). It is far too easy to lose updates when someone sends you a file that on the surface "looks like" the same one you had (same file name, mostly looks the same, you cannot see the changes because they were not marked or tracked properly etc). Or you overwrote B's updates with C's file when you saved the attachment. And even worse - if it is not being reviewed closely enough, that might not be caught until it is too late. Sound familiar?
There are many possible solutions to this of course - but I always like to think that simpler is better, because if you make it simple, people might actually use it. There are a lot of very expensive, fancy systems people just don't use - or don't use properly because they are difficult or tedious to work with.
Note that this is not a discussion on the "track changes" features of various document programs, which of course you should be using as well to track the internal updates. This is one level up from that, but just as important.