To give a better answer, the blue Atmospheres are exactly that, they show on planets (in description, between Surface Temp and Volcanism) the Surface Pressure in Atmospheres. This appears to have NOTHING to do with landing, as landable Horizon planets can have them or none at all.
I thought this might play into surface temp, the im not so sure after looking at it for a bit. Seems like more noise in the system map UI (whether welcomed or not, up to the player). If there was meaningful gameplay decisions that could be gleaned from knowing it has an atmosphere, Id like to know that.