There’s many good reasons.
Most commonly you batch to make life easier when you have friends coming over and you don’t want to get stuck making drinks all night.
You simply enjoy having your favourite cocktail in the fridge ready to serve as and when the urge grabs you!
There are some techniques and recipes that can only be done by batching, such as ageing or carbonation.
You need a lot less ice, or even no ice at all! This is a big plus in the real world when you‘re entertaining. The process of making cocktails with traditional techniques can require a LOT of ice.
Many drinks, especially stiff, booze forward cocktails like a Martini or Manhattan, will even improve with time resting in a bottle. Like slow cooked food, all the flavours become more rounded and integrated as the molecules interact with each other.
Downsides? Not many. Only the fact that the drink is set. So, if you batch Martinis for a party then you have predetermined how Dry everyone is drinking their Martinis (i.e. the ratio of Gin to Dry Vermouth).
Carefully measure out ingredients and combine together in a clean mixing jug.
Water that would usually be added by techniques like stirring or shaking, is measured out and added with the rest of the ingredients.
The cool thing about making drinks this way, is that once you have measured out your ingredients, bottled and chilled them, the rest is enjoyment. There’s nothing left to do other than pour the cocktail into your glass, garnish and serve. If you still want to go to the trouble of shaking or stirring, you can save some time by mixing ingredients together ahead of time, but you are only making your life slightly easier. Better to go the whole way and include the water in your batch.
The amount of water you need to add will either be included in the recipe you are following, or you will have to work it out yourself. Here’s a way to do that: