Even for technical documents, if you expect your target audience to be familiar with the domain, nobody remembers every abbreviation every time. It also helps new readers to quickly familiarize themselves with the material (and might even help a few understand more than they would have done without expanded abbreviations).
- for the readers who already know the abbreviation, it's a waste of space and time
- for readers who don't, the words won't tell them much either - Transport Layer Security tells you about nothing about what TLS really is - besides, for those who are really interested, they can always look it up on Wikipedia (that way, they will actually understand it).
Doesn't it? Even a complete tech-illiterate can glean some meaning from the word "security".