The Append tool is very similar to the Merge tool. It also combines data from multiple layers or tables into one. The big difference between these two tools is that Append is one of the few geoprocessing tools that changes the input data. It will add features or records to the target input.
You might use the Append tool if you have an existing layer of information and you just need to add newly acquired data to it. For example, continuing with the previous hydrant example, after merging the hydrants from the first four days, you collected some data. Now, you go out and collect more hydrant locations. You wish to add those newly collected hydrants to the merged layer to create.
The Append tool would work in this case. It would continue to add the newly collected location to the existing data layer. It would not keep creating new layers that you would need to manage. This is illustrated in the following screenshot:
