"We need to have competition for places," said Mackay.
"You need the players to be on their toes because they know someone is ready to come in and take their place."
He joins Joe Mason, Robert Earnshaw, Rudy Gestede and fellow new signing Etien Velikonja.
"He's (Helguson) a very experienced player," added Mackay, who has also signed Joe Lewis, Jordon Mutch, and Kim Bo-Kyung this summer.
"There's an experience to him and professionalism to him that I know very well, and I think he will be a great addition to our group.
"He's very robust, a very good professional, brings high standards with him and certainly has an eye for goal.
"These are all qualities that attracted me to him."
Helguson scored eight Premier League goals last season including the winner over Chelsea.