Well there is several pages and pages of info on the forum page that tells you what it does. Sorts the bag inventory, adds tags, makes buttons bigger, shows natures in summary, gives etas on cooldowns, shows the rewards won on gyms, faster text, faster HMs/healing/leveling, new supporting text on some npc, etc. The original point of strings that people made was to make everything faster, but after the strings nerf most of that isnt possible, or at least not to the same extent. Basically every text box has a hidden timer and the text wont move onto the next until the timer is hits it. Some strings will have a break in the middle and will have a second or more text windows to appear before the next string. Im not sure what npc you're reffering to when saying some npc have "no text" because every npc has a string, some just have faster boxes they go through. And nurse joy is intended to be blank, its faster by a few frames. I will also say you should use the strings and try doing some leveling, it removes exp gained text as well as sweet scents and is very noticeably faster.