Because it is not just to change the temperature. You want to "rinse" your grains during your sparge to extract sugars. Think of it like this: After your mash is done, sugars like to "cling" to the grains, so they have to be rinsed off.
There are essentially two types of sparge:
1. Fly Sparge. This is where the grains are kept in a mash tun. It is drained from the bottom while being filled at the top, so there is constantly water running over the grains inside and extracting the sugars.
2. Batch Sparge. This is pretty much what we are doing with this method. In a true mash tun, the wort is drained from the bottom completely, then more water is added, the grain is mixed up and it is drained again.
3. There is another method: No sparge aka BIAB (brew in a bag). This is where a high concentration of water is used with your grains and then the bag is lifted up and left to drain. This method can work well for some, but I'm not sure how high you can go with the water without problems in the mash and I'm not convinced effiency won't suffer from this method.
I do not think that adding more water to the initial boil pot would work well in extracting the sugar from the grains. It also comes into how much water you can fit in a pot. This method is designed for stovetop, where many people use very small pots.
There are essentially two types of sparge:
1. Fly Sparge. This is where the grains are kept in a mash tun. It is drained from the bottom while being filled at the top, so there is constantly water running over the grains inside and extracting the sugars.
2. Batch Sparge. This is pretty much what we are doing with this method. In a true mash tun, the wort is drained from the bottom completely, then more water is added, the grain is mixed up and it is drained again.
3. There is another method: No sparge aka BIAB (brew in a bag). This is where a high concentration of water is used with your grains and then the bag is lifted up and left to drain. This method can work well for some, but I'm not sure how high you can go with the water without problems in the mash and I'm not convinced effiency won't suffer from this method.
I do not think that adding more water to the initial boil pot would work well in extracting the sugar from the grains. It also comes into how much water you can fit in a pot. This method is designed for stovetop, where many people use very small pots.