This tool is used only in cases where streams flow through a lake and you want to force all cells within the cells to connect to the streams as quickly as possible. If you do not have streams in your lake, you do not need to use this tool.
Another case to consider is whether your lake should be considered as a sink - in that case you would have to use the Adjust Flow Direction in Sinks tool so that all cells in the lake flow toward the same sink point.
Christine Dartiguenave
Esri Water Resources Team