Connecting batteries in series doubles the voltage while maintaining the same amount of current. Series circuits do not overheat easily. This makes them very useful in the case of something that might be around a potentially flammable source, like dry plants or cloth.
Connecting batteries in series increases voltage but do not increase overall amp-hour capacity. All batteries in a series bank must have the same amp-hour rating. Connecting batteries in parallel increases total current capacity by decreasing total resistance, and it also increases overall amp-hour capacity.