The purpose of a small number of US troops would be to stand by the Kurds who helped defeat ISIS. The agreement by the US, Turkey and others provides for a buffer zone in northern Syria between the Kurds and Turkey. A US presence should dissuade Turkey from violating the agreement and attacking the Kurds beyond the agreed upon buffer.
Turkey is no longer a good NATO ally and Erdogan has his own agenda and visions of new caliphate.
Tripwire troops. The purpose is to force US military involvement should the Russians, Iranians, Syrians, or Turks attack. We had similar reasons to station troops in West Berlin.