I am installing a sandpoint to use for filling up my swimming pool and watering my lawn. Is it necessary to have a bladder tank?

No.

The only reason to have a pressure tank would be if your pump is delivering more volume than your sprinkler system can handle. I would put a pressure gauge on the system and monitor that. If the pressure starts staying fairly high (60# or better), then I'd put on a tank just to keep from overworking my pump.