Most fly lines today come with a welded loop on the end to make it easy to attach a leader. Eventually that loop will wear out and you’ll need to add a new one. There may also be times when you’ll want to replace that welded loop with a stronger loop, in particular if you chase big game fish such as tarpon.
The answer to both situations is to cut off that welded loop and make a whip finish loop in your fly line as shown in this video from FlyCarpin.
Enjoy!