Comment: More info is needed. Is the on-line filter on the input or output side? Normally, autotransformers do not need on-line filters. Therefore, there might be other reasons such as:
1. To improve quality of power supply, e.g. mitigate voltage or current or both power supply side harmonics; and
2. To mitigate voltage or current or both distortions caused by nonlinear loads.
3. If the filter is small, it could suppress electromagnetic interference, i.e. it could be an EMI filter.
