From the Super Mario Wiki
A ByeBye Balloon is an object in Wario World. It is essentially a Glue Globe with a bunch of colorful balloons attached to it. They are always found near the entrances to the bosses of levels. Wario could use the ByeBye Balloon to go back to the start of the level he was in and collect any missed treasures.
If Wario beats the boss of the level and returns to the same level, a ByeBye Balloon will be near the start to take him to the midpoint of the level. This allows Wario to collect missed treasures faster.