What is the purpose of the dungeon in Future Hyrule Castle?

After you get transported to the future in Cadence of Hyrule, there are just a few options for where to go:

• Go down the stairs and fight your way through a typical dungeon.
• Go into the main castle entrance and fight Ganon.
• Go into the side entrance to solve a puzzle for an item.

It seems strange that you can fight Ganon without going through the dungeon first. Upon completing the dungeon, it leads out to the right castle wall. Although it appears that you couldn't get to this place except by going through the dungeon, it doesn't seem like there is anything to do here. Is there an item or a secret that requires completing the dungeon?

I can't remember if we received any of the unique/legendary weapons in this dungeon; but even if we did; we could have just warped out after getting all items instead of actually completing the dungeon.

Prior to the Symphony of the Mask DLC, there was only one path to reach Ganon: Collect the four instruments from Octavo's minions, defeat Octavo, follow him to the future, and proceed through the future dungeon. Only after beating the future dungeon could you access Ganon.

With the Symphony of the Mask DLC, you are given a second alternate path to reach Ganon: Dig through the cave wall in the Lost Woods, taking you to the Deku Tree in future Hyrule. In future Hyrule, you can complete the two dungeons there to obtain the Bongos and Synth. Unlocking the future Hyrule front gate with these two instruments allows you to reach Ganon directly without going through the future dungeon.