Abbreviations are short forms: they save the lowly scribe (picture the medieval monk copying entire books by candlelight) from having to write the same lengthy term repeatedly. In a period when one sheet of paper cost more than a whole meal, abbreviations also saved space.
In the modern technological world, there’s no real excuse to use abbreviations anymore, other than we’ve become accustomed to them. When speaking, we’ll occasionally use an abbreviation (Where did you get your PhD?) but will more often say the full word. It’s only when writing that we’re seriously inclined to make things short and sweet. However, when writing, it’s important that your reader not get so bogged down in figuring out the short forms that they forget what they’re reading.