Is the question for connection of loads like bulbs, fans, refrigerator, tv and such gadgets at home ?
All the electric appliances, and other elements are usually designed for 220-240 V supply. Each load is connected across the mains in parallel with the other loads with some separator. The reason is that if one appliance fails or if there is excess load in one circuit, or if there is a short circuit in one circuit these do not affect the other appliances.
If they are connected in series, then the voltage across the appliances becomes variable. The appliances may not function properly if there are too many connected in one circuit. More importantly, if one appliance fails (opens the circuit) , then others also are switched off. If one appliance becomes a short circuit, then all other appliances are affected due to excess current.
if you are mentioning about connection of batteries in parallel or series?
if there is more current needed, then we connect them in parallel. In case the external resistance is less than the internal resistance of the batteries , we connect them in parallel.