An auto transformer is a type of electrical transformer that uses a single winding to act as both the primary and the secondary winding, with a portion of the winding common to both. This type of transformer is widely used for various applications due to its efficiency and cost-effectiveness.

Let's analyze the options given:

1. Reduce insulation: Although an autotransformer may use less insulation compared to a two-winding transformer for the same voltage level, the primary purpose of tapping in an autotransformer is not primarily to reduce insulation.
2. Increase magnetic losses: No electrical device is designed with the purpose of increasing losses. In fact, transformers are generally designed to minimize losses.
3. Regulate voltage output: This is the correct option. The primary purpose of having a tapping in an autotransformer is to adjust the voltage output. By changing the location of the tap on the winding, the number of turns and thus the voltage can be varied, allowing for control over the output voltage.
4. Separate windings: In an autotransformer, the windings are not separate; they are partially common. Thus, tapping cannot be used to separate windings, which is a characteristic of traditional transformers with distinct primary and secondary windings.

Therefore, the correct answer is that the tapping in an auto transformer is used to regulate voltage output.