Sometimes you do not want to wipe everything. You want only the posts that mention one topic, or only the ones from a certain stretch of time. Here is how to do surgical deletes on X instead of nuking your whole history.
Maybe it is an old job, an ex's name, a phase you grew out of, or a single topic you no longer want tied to your name. X search can help you find these, but it will not delete them in bulk for you. A keyword filter does: it reads the text of each of your posts and removes only the ones that contain the words you choose, leaving everything else untouched.
This is how you "delete your first year on X" or "clear everything older than a year" without touching your recent posts. A date filter checks each post's timestamp and only removes the ones inside the window you set. Set just an end date to clear everything older than that, or a start and end date for a specific period.
X loads roughly your 3,200 most recent posts at a time. So filtering works perfectly within that recent window. To reach truly old posts (for example the very start of a large, years-old account), you delete the newer ones first so the older ones load in, then run the filter again. For most accounts this is not an issue.