This cookbook modifies file and directory resources in a resource collection to be rooted somewehere else in the filesystem as specified in the attribute node['treegraft']['root']. It does not matter where in the run list that treegraft::default appears, because it uses a Chef event handler that runs before the converge begins.
Suppose you have a cookbook/run list with this resource:
file '/empty_file.txt' do
action :touch
end
If you include treegraft::default in the runlist and set node['treegraft']['root'] to /arbitrary, then Chef will touch the file /arbitrary/empty_file.txt.
The use case for which this cookbook was written is that of using a cookbook to manage files in source code repositories, which are typically not rooted at /.
This cookbook makes available a method Treegraft.lookup_path that can be used to create a list of places that a resources source property should check. See its source or tests for more details.
The event handler only modifies Chef::Resource::File and Chef::Resource::Directory (and their subclasses) resources. Other resources have a path property, but I haven't identified use cases for modifying their locations.