Unlocking flying for the older zones is great as many of the job quests between DoW/M and L/H have to go visit those areas multiple times. Especially when you get to the gathering jobs you will find yourself constantly visiting those zones for the quests but also to work on leves for the materials that are there.

Also if you want to do hunts it's heavily regarded you unlock those currents as hunt trains will go visit HW zones for A/S ranks. If you're on foot/mounted on ground you may not get there in time for the pulls as opposed to flying.