Question
Consider ${R_1},{R_2},{R_3},...$ be the internal resistances of each cell. Final resistance, ${R_f}$ formed by connecting cells in series and parallel have different relationships with the individual resistances. When the cells are connected in parallel, then ${R_f}$is calculated as follows:
$\dfrac{1}{{{R_f}}} = \dfrac{1}{{{R_1}}} + \dfrac{1}{{{R_2}}} + \dfrac{1}{{{R_3}}} + ...$
This way ${R_f}$ estimated is lesser than when connected in series, where the final resistance is simply numerical addition of all the resistances. So the purpose of connecting Primary cells in parallel is to decrease the final resistance.