This retains lines beginning with the text you specify and discards the rest.
Why the start position is useful
Structure lives at the beginning of many values. URL paths start with their section, product codes with their category prefix, log lines with their level or timestamp.
The main use
Isolating a site section from a URL list. Keeping lines starting with /blog/ gives exactly the blog, with no risk of matching the word elsewhere in a path.
More precise than an open search
Anchoring to the start removes the false matches that an unanchored search would collect from the middle of longer values.
Try it: Keep Lines Starting With on SeoWolf's Notepad.