PRS rifles want the balance just forward of the mag well for barricade balance, so you start with the biggest heaviest chunk of high-pressure steel target pipe you can get forward, then work your way back to a lightweight stock.
For a rifle you're carrying all day, lightweight everything would be wonderful. Some target disciplines put you under a weight limit so you have to budget, more grams where it helps your score, scrimp grams away from things that shouldn't hurt you. All depends what the rifle's for, and there are a lot of different niche applications people tune theirs into.