The lower the sensor temperature the less noise, especially for longer (DSO) exposures. I think that cooling is even of some benefit, though not essential, for planetary imaging, allowing (I think) the gain to be increased to set shorter exposure times, for "lucky imaging".
Going below zero doesn't in itself mean that dew will form, although air can carry much less water at 0 and below than at "normal" temperatures. As mentioned above, cooled cameras have various precautions to counter dew formation such as dew heaters (ironically) and desiccant tablets.
EDIT: An often stated benefit of cooling fir DSO imaging is that it allows for lights and darks to be exposed at a constant/controlled temperature,
Edited by Rac19, 24 September 2021 - 08:26 PM.