The material and thickness of your battery tab may mean that projections are required in order to optimise the welding result.
when batttery tabs are too thick or when you are welding a conductive material, having no projection can lead to only small amounts of heat being generated in the weld area. This leads to a poor welding result. This can be observed in the left-hand image.
The inclusion of a projection in your battery tab can improve your welding results by directing the current through a much smaller area, thus generating much more heat, leading to better welding results. This can be observed on the image on the right-hand side.