Snowball thinks that the animals will all have much easier lives if they are able to build the windmill. This is why he wants to build it. Napoleon disagrees with Snowball -- he thinks that they should just concentrate on growing food.
Snowball says that building the windmill will allow the animals to have electricity. He says this will be good for them because it will allow them, for example, to have their stalls be warm in winter. It will also help them by saving work because there will be some sorts of machines that can run on the electricity.
You can find more details in Chapter 5.
In Ch.5 we read that in spite of Napoleon's objections Snowball successfully completed his plans for the windmill. One Sunday all the animals gathered together to decide whether to start work on the windmill or not. After Napoleon had once again expressed his disapproval to Snowball's plans Snowball made a passionate defense of his plans for the windmill. He said that the windmill would be used to generate electricity which in turn would liberate the animals from the drudgery of hard labor on the farm and would make life more comfortable for them:
In glowing sentences he [Snowball] painted a picture of Animal Farm as it might be when sordid labour was lifted from the animals' backs. His imagination had now run far beyond chaff-cutters and turnip-slicers. Electricity, he said, could operate threshing machines, ploughs, harrows, rollers, and reapers and binders, besides supplying every stall with its own electric light, hot and cold water, and an electric heater.