Answer
DEGIRO is a financial institution and as such is legally obliged to inform the Dutch tax authorities of the countries in which you are considered a tax resident. The Dutch government cooperates with many other countries to counter tax evasion and therefore requests this information from financial institutions. This is further explained below in the questions ‘What are Common Reporting Standards (CRS)?’ and ‘What is the Foreign Account Tax Compliance Act (FATCA)?’.
Moreover, we may ask your tax residency to determine whether a reduced withholding tax rate applies under a tax treaty established between your residence country and the source country (for example, the tax treaty with the US).