Battery Box Pieces

[Bierkan]

Not being that familiar with this, I'm trying to figure out why there is a separate tray in addition to the tray on the bracket. Is the stand alone tray smaller to help hold the battery in place? Or is the tray a replacement piece to deal with corrosion?

My truck doesn't have one currently. The battery is under the hood up by the radiator and the truck doesn't have inner fenders, yet. When I add them, I'll have to relo the battery and I'm thinking putting it back under the cab.

What are the pros and cons of having the battery in the stock location?