This tool can be used for merging multiple designer objects or connected faces to an object or a face.
How to use
Select objects you want to merge.
And then click the "Merge" Button.
Then all selected objects will be merged to the last selected object.
Face (From 3.6.7)
There is a designer object created with the Cube Editor as follows.
Select faces you want to merge.
And then click the "Merge" button. The faces which are connected and have same material ID and on the same plane will be merged to one face as follows.