During the day they have a cooling effect by reflecting/blocking some light.
The optimal is not to eliminate them entirely, but to create them only early in the day so that they dissipate by sunset.
> Recent estimates show that contrails cause about the same amount of warming each year as all the CO₂ emitted by all flights from 1940 to 2018.
For most flights today though, if you only count the CO2 produced you're understanding the climate damage by about half.