They’re a tripwire, and they can act as Forward Air Controllers and bring down H*** from above in large quantities.
A trip wire to prevent an offensive is one thing. Turkey is in NATO how crazy is it going to be for the United States to get into a hot war with Turkish troops in Syria? It seems to me that Turkey has always hated the Kurds and has always had to live with them next door. They aren’t going to massacre or wipe out the Kurds. More likely they are going to get themselves into a mess.
Sure they can, but doing so is utterly unrealistic. What you are saying is that some forward air controller in Syria is going to call for air attacks against Turkey, a NATO ally. And then presumably US aircraft take off from Incirlik in Turkey, fly across Turkish airspace, and then bomb Turkish units. And then they turn around, shoot down any Turkish air force assets in the way and fly back to Incirlik where they US pilots go to bed in their barracks.
I haven't seen anyone make a realistic case about how any US military asset would be used against Turkey. Everyone acts like we could just start shooting down Turkish jets, or attacking their army and they wouldn't do anything in response - like attack our planes and pilots based in their own country.
If Turkey decides to attack Syria we could join forces with the Syrian government, and give them armaments to drop from their fighter jets, but I suspect there are not many politicians in DC who think arming Assad is a good plan.