the GB Image Block has far more styling controls then the core block does. And it supports dynamic data contents.
We therefore recommend the GB Block is used for designing more complex layouts or for using Dynamic Data content in a block element templates or GB Query Loop.
But for adding images inside a post content, we would recommend you stick with the core block.
So there may be a need for both, you may just want your users to only access the core block, whilst the site admin / designer / developer may want both.