There are a number of reasons why fanless devices are better than fan cooled ones and some of them are not strictly related with heath and air ventilation.
Less Noise
This can be obvious as fans, by definition, move air therefore generate noise.
Improved Shock and Vibration Resistance
Fans add moving parts as well as additional connectors to the device hence increasing points that can fail when the equipment is subject to shock and vibrations.
Less Power consumption
This advantage is not as straightforward as the previous one because it is a side effect of fanless design. In fact, in order to be fanless, devices need to generate less heath and therefore consume less power.
Increased reliability
Needless to say that moving parts like fans are more likely to fail than static components.
Furthermore, when a fan stops working or reduces its efficiency the internal temperature will increase with negative effects on components reliability.
However, even when fans operate properly, the air they pull or push inside the equipment contains dust, grease particles and small debris that stick to components modifying their working conditions: reducing heath transfer, increasing corrosion, etc.
Safety in Sterile Environments
The dust accumulated by fans inside devices can be a favorable environment for bacteria growth reducing the sterility of the surrounding environment. For this reason fanless operation is required in medical environments.
Compact form factor
Most fanless products are smaller because they rely on metal heath sinks rather than air circulation, therefore the overall size of fanless devices tend to be smaller.