It is only for certain types of content, and it needs to be obvious what it does. For long copy which is only going to interest certain visitors, show/hide (with an excerpt finishing mid-sentence and an invitation to click to see more) may help – reader gets an overview with the option of more if interested and remains on the page. If they choose to read they are probably less likely to get bored and leave, missing that key fact further down. Also good for Q&A. I hope I have that right :) @silvabokis is the ux specialist!