Why do we need to join these cells together and how does circuit pass through it.and how does it help
In series, cells are joined end to end so that the same current flows through each cell. In case if the cells are connected in series the emf of the battery is connected to the sum of the emf of the individual cells. Suppose we have multiple cells and they are arranged in such a way that the positive terminal of one cell is connected to the negative terminal of the another and then again the negative terminal is connected to the positive terminal and so on, then we can that the cell is connected in series.
If E is the overall emf of the battery combined with n number cells and E1, E2, E3 , En are the emfs of individual cells.
Then E1 + E2 + E3 + …….En
Wiring batteries in series provides a higher system voltage resulting in a lower system current. Low current indicates that you can use thinner wiring and suffer less voltage drop in the system.