The Inject Attributes component is used to inject an existing ShapeDiver Attributes object into another object in Grasshopper. Typically, the Attributes Object is built using the Construct Attributes component or extracted from imported geometry.
Injecting Attributes in an object which already contains injected Attributes will override the existing ones, including key which are not contained in the new Attributes. If the goal is to add values to an existing Attributes Object, use the Add Key-Value Pair component.