Meaning of Add and Test in ALU

The ALUF[2:0] includes addtion. However , it seems that the ALU also gets an add signal input. Can you please explain why ?

\texttt{ALUF}[2:0] is derived directly from the opcode. So when the instruction is \texttt{add} or \texttt{addi}, the ALU can be directed to perform addition using it. However, addition also needs to be done during the execution of other instructions, e.g. incrementing PC. In those cases the \texttt{add} signal overrides \texttt{ALUF} and directs the ALU to performs addition.