Abbreviations often reduce reader work and save space. For example, all readers of technical reports should recognize that ml refers to milliliter and cm refers to centimeter. That these abbreviations save space is obvious but how do they reduce work? Any reduction in number of letters that does not reduce comprehension reduces work because it is work to look at and comprehend letters. Fewer letters (and numbers) means less work for the reader.
Other abbreviations are less commonly used but are still effective. An example would be PCR for polymerase chain reaction. In science papers, PCR is now used without definition because virtually all readers know what PCR means. In a popular article, however, the writer would probably not assume that the reader would know the meaning of PCR and would probably introduce the full term and then the abbreviation. For example: "The scientists used a technique called polymerase chain reaction (PCR) to determine whether the virus was spreading from domestic to wild birds." After defining PCR, the writer would use it instead of polymerase chain reaction throughout the article. As with cm and ml, the abbreviation PCR conveys information more efficiently than the full words.
Although writers often use abbreviations to reduce reader work and save space, they sometimes use abbreviations to impress and inflate, i.e., they use abbreviations as jargon. The use of an abbreviation suggests that the abbreviated term is sufficiently important and is used sufficiently often to warrant abbreviation. This is clearly the case with cm, ml, or PCR. But what about BCAs? Before I retired, I studied biological control (as opposed to chemical control) of agricultural pests. In other words, I tried to control pests with biological control agents but I never used the abbreviation BCAs. Why not? In my view, the term is insufficiently important and insufficiently common to warrant abbreviation. I rejected the notion that biological control agents should be enshrined in the abbreviation BCAs.
Using abbreviations is also trendy and cute. Consider e-mail language and text messaging (lol, kma). If you want to seem trendy and cute, then use lots of abbreviations (and kma).
When you see an unfamiliar abbreviation, you should ask yourself whether the author is trying to help you understand or whether he or she is using the abbreviation to inflate and mystify. In your own writing, use abbreviations when they will help the reader (or when you do not respect the reader and are trying to convince him or her to buy something from you).
Abbreviations are also ineffective when they are introduced and then not used or are used inconsistently. If you introduce an abbreviation in a paper, use it throughout the rest of the paper. And abbreviations are ineffective when they are accompanied by too many other abbreviations, as in "EPNs might be effective BCAs of PPNs" (Entomopathogenic nematodes might be effective biological control agents of plant-parasitic nematodes). The full length version is certainly longer and lacks poetic impact but at least doesn't sound like a joke.