Perhaps the most widely accepted argument for harmonization involves convergence in the definition of product value or income for tax purposes.
Such tax base harmonization would contribute to transparency for economic decision-making and, thus, to improved efficiency in resource allocation.
Tax harmonization doesn't automatically lead to the formation of a fiscal union, the second part involving much larger scale project that includes fiscal transfers, a fully harmonized legislation and maybe some supervising institutions, beside a long-run agreement.
Excise duties are also subject to minimum rates, based on Articles 191-192 of the Treaty on the Functioning of the European Union (TFEU).
This treaty base allows the Council and the Parliament to take decisions, including on taxes, to protect human health, safeguard the environment and promote a “rational utilization of natural resources”.
[4] In 1990, the Parent-subsidiary directive tackled the issue of double taxation of repatriated profits by a mother company from its subsidiaries.4 Member states are requested either to exempt repatriated profits, or to deduct taxes already paid by the affiliates from the mother's tax bill (partial credit system).