When you use an acronym or an abbreviation, you’re assuming that the other person knows what you mean.
It may be obvious to you, but is it obvious to the other person? Not necessarily.
Even if the other person does know what you mean, unless the acronym has been fully internalized, it will require a mental translation. This unnecessary processing requires extra time and detracts from the point you’re trying to make.
In addition, a message is only written once, and could be read many times. Therefore, logically, it’s more efficient if the writer puts in additional effort to be as clear as possible and save other people’s time.
Furthermore, the acronyms can conflict with other acronyms. Take for example, one of my current projects I’m working on: Content Services, which is commonly referred to as CS.
CS could mean many things. It could mean computer science, customer support, Creative Suite (by Adobe), case-sensitive or client-server.
If you’re a gamer, the first thing that comes to mind may be Counter Strike. If you’re into psychology, like me, you may also think of cognitive science.
You probably don’t know conclusively what the other person’s areas of expertise/interest are. You also don’t know if your communication will be forwarded to other people (unless it’s highly confidential, but this usually isn’t the case).
If you want to abbreviate something because you want to mention it multiple times, the first time you mention it, use full words and put the abbreviation in parenthesis, then use the abbreviation.
P.S. I mentioned earlier that some acronyms and abbreviations are usually internalized. For example, I wrote P.S. instead of postscriptum because people are more familiar with the shortened version. :)