Pinning a tweet means you want to keep displaying a specific tweet on your Twitter profile. This tweet appears at the top no matter how many tweets you post afterward. This article explains how to pin a tweet on Twitter.
Generally speaking, the more you tweet, the further your past tweets move down your profile page. However, Twitter makes it possible to do this so that it stays at the top of your page until you remove it.
When posting on your Twitter profile every day, your older tweets fall on the list and become less visible. So the perfect reason you’re pinning Tweets is to keep seeing the most important post on your timeline. However, any tweets you post afterward don’t change your pinned tweet’s position.