Well, It could be useful sometimes to write what you want on Mac Text Editor first or maybe you want to share something you have already on a txt file on social media networks, an email or even on iMessage or SMS. Its very easy just highlight the text you want to share then right click and select: Share. You will get a sub-menu includes all available apps / accounts yo can share on.
You may be asked to setup your accounts if you didn’t do it before. For example if you try to share on Facebook and you didn’t setup your Facebook account on your MAC before you will be asked to set it up so you can share on the first time only.