There's a concept called "max TDP" which essentially means how much heat the cooling system (fans, heat sinks, heat pipes and case) will need to dissipate in the worst realistic case.

For the CPU in the base model 2013 Macbook Air (the Core i5-4250U), max TDP is 15 watts.

For my 2011 Sandy Bridge base model Mac mini, max TDP is 35 watts. Most desktop gaming PCs have max TDPs over 100 watts if the heat generated by the discrete video card is included.

Max TDP for the CPU in this Zotac box is even lower than the 15 watts on the 2013 MBA. To achieve such a low max TDP, the clock needs to run slower, the chip needs to use lower voltages and other compromises need to be made.

My guess is that one place such a low TDP is desirable is in small tablets (or even smartphones??) where space considerations make cooling thing difficult even with a fan.

And of course it is desirable in fanless applications. Having just one fan, like an Intel NUC has or a Mac mini has, greatly increases the amount of heat the system can dissipate. The 2011 Mac mini server edition, for example, has a CPU with a max TDP of 45 watts and just one fan. (The Mac mini's all-aluminum case really helps here.)

But sometime even one fan is a bad idea: e.g., in boxes designed for dusty conditions, e.g., in a factory, or a remote location where it is difficult for someone to come by to clean out the insides of the system or to notice (by the noises it makes) when the fan is about to fail.

Note that fanless systems with SSDs like this Zotac box have no moving parts, and that really helps with reliable un-attended operation -- and with being quiet.

EDIT: "significantly less" changed to "even lower" to reflect the information in the reply.