Many come to the United States on various work Visas and eventually bring their spouse and children to join them. Now these family members may not qualify for Social Security Numbers (SSN) and they have to apply for an Individual Taxpayer Identification Number (ITIN). The member of the family here on the work visa and consequentially an SSN can claim dependency credits and also be able to file jointly on their U.S. tax return with their spouse if conditions have been met and they qualify.
The 2017 Tax Cuts and Jobs Act now requires all applicants for Employer Identification Numbers to have either a Social Security Number or an ITIN. Non-resident individuals doing business in the US as a partner in a US partnership also require an ITIN to file their US tax returns and to be able to claim back taxes paid on their behalf by the US partnership.