They are, of course, trying to sell you stuff. However, if you have a mesh safety cover, getting some chlorine into the pool early can prevent opening to a green pool. It may be VERY green if you open a few weeks after the weather gets warm. I string a floater in the middle of the deep end of the pool, open slightly, shortly after the danger of freezing is over. Last year I neglected to do so and dumped 15 to 20 gallons of bleach in the pool over about 4 days before the the chlorine level stabilized and the algae was gone. Usually, I use 4 or 5 gallons for shocking if I have used the floater. It also saves on brushing and backwashing to get rid of the algae.
If you have a solid cover, the danger of a serious algae bloom is significantly less because of the lack of sunlight.
Chris