Why does the US still have forces in Syria?

US forces have targeted a base operated by Iranian allies in eastern Syria after an attack near a US base last week.
The United States’ air attacks on targets it says were associated with Iran’s Islamic Revolutionary Guard Corps (IRGC) in eastern Syria have put the spotlight once again on the continued presence of US forces in the country.
In recent years, Washington has pulled back from its previously more extensive role in the country, but has stayed on in a limited capacity in some areas, ostensibly to fight ISIL (ISIS) and limit Iran’s influence in the country. The eastern Syria raids announced on Wednesday appear to be part of the latter, with Central Command spokesperson Colonel Joe Buccino saying the strikes “were necessary to protect and defend US personnel”, and a response to an August 15 attack targeting US forces that was allegedly launched by Iranian-backed militias. When did the US militarily involve itself in Syria? Despite indicating its support for the Syrian opposition shortly after the beginning of the uprising against Syrian President Bashar al-Assad, the US’s first major military involvement in Syria was against ISIL.
In September 2014, a US-led coalition launched air raids against ISIL in Syria, as part of a campaign it was already conducting in Iraq. Source: aljazeera.com