Magento's built-in tax rules engine works well as long as tax rates can be expressed through a manageable number of tax classes, customer groups, and regions. Once a merchant sells across many US states with thousands of local tax jurisdictions, or ships cross border within the EU under the One Stop Shop scheme, static rule configuration hits a clear wall. External tax services keep such complex scenarios continuously up to date, but need to be wired in cleanly through TaxCalculationInterface, including a deliberate caching strategy so every price display does not turn into an extra external API call.