Currently for the different variable-types in Puppet/Ansible/Salt only Arrays and Hashes support the Option to Merge (Merge, Merge-with-default, Merge-remove-duplicates).
Depending on how you define your variables, you might want to use other types as well.

For example if you build a puppet-class where you declare only one variable of type "yaml" to reduce the amount of variables and to define everything you need per class in just one variable. In this case you often end up that in certain environments/matchers you want to add something without having to copy&paste everything. In this case a merge-option for "yaml" would be great.

Since in the current case it is puppet, it applies to salt and ansible as well (and maybe other? Host-parameter?) and it might be useful as well for other types, such as "json".
For this I put it in the general foreman-project and not three times for the different configuration-management-plugins.