Earlier quoted context omitted.
Several reasons. Etiquette. Less reason mangle the words of others (arbitrarily cutting something out of larger context can also easily mangle). Let's your reader see how you understood the communication, not just that you found it somewhere and copy-pasted it in.
Etiquette is circular logic. A larger quote would still be arbitrarily cut out of a larger context. If you're extending past the good part purely to get a subject I'd even say it ends up more arbitrary. Unless you're taking a test, you don't need to prove you understood it, and if your reader lacks confidence in your understanding then paraphrasing makes the problem worse! Instead of merely worrying you missed import…
I was tempted to reply to this by throwing etiquette out of window, but that would get me downvoted to hell. Because there is an etiquette.
> Paraphrasing can mangle words so easily. I'm baffled why you would imply otherwise.
When I paraphrase someone, they're my words. Not the original author's. I take responsibility for them.
> And your edit just makes you look stupid.
You're right down there with me. Welcome.