This keeps every line where the specified text appears anywhere and discards the rest.
A filter, not a search
Searching shows you matches while leaving the list intact. This produces a new, smaller list containing only the matches, ready for the next operation.
Where it is used
Pulling every URL under a directory, every keyword containing a product name, or every row mentioning a competitor.
Chain filters to narrow further
Applying several keep filters in sequence progressively narrows a list. Each pass is simple and independently verifiable, which is easier to reason about than one complex expression.
Try it: Keep Lines Containing on SeoWolf's Notepad.