What does this mod do?

The mod adds a new subcommand for the /item command. It looks like this:

/item HoldingType <type>

↑ This will change the holding for an item in the main hand

/item HoldingType <type> <hand>

↑ Same as above, but will allow you to customize your specific hand.

Why do I need this mod?

You will be able to customize item holdings for your own mod, datapack or map. My mod also allows you to customize items from other mods. As an example, see screenshots, I showed there how you can use the mod on the example of a lamp from my other mod.