Superior performance per watt.
In fact not only regarding performance per watt, but also performance per dollar. It's just that ARM designs for lowest power consumption while Intel/AMD design for maximum perfomance.
* Performance per dollar operating cost (performance per watt is closest to this)
* Performance per dollar capital expenditure (important for desktop systems, where operating costs are low)
* Performance per dollar TCO (sum of the above two)
The third one is the important one.
Why ARM?
How does the ISA impact the aforementioned criteria?
Why would a phones demand a different ISA?
ARM cores are typically slower in absolute terms than Intel cores, but at a given level of power, you can run more of them.
The differences between modern ARM cpus and modern x86 have less to do with the ISA itself and more to do with the way ARM cpus have been designed to be low-power for decades and have worked their way up the performance scale, while x86 has been designed for performance and has only lately been emphasizing low power. These lead to different design points.
The x86 ISA fundamentally takes more silicon to implement than ARM. More gates = more power.
This is not strictly true, the processor throughput also matters.
Total Power consumed = Power consumed by gates * Time taken to finish the job
Total energy consumed = Power * Time
For ARMv7 vs x86, yes, x86 just destroys ARMv7 (Cortex A15 etc.) in double (float64) performance.
While I do think x86 is still faster vs ARMv8, the gap is likely much less per GHz, because ARMv8 Neon now supports doubles much like SSE. Of course Haswell has wider AVX (256-bit) and ability to issue two 256-bit wide FMAs per cycle (16 float64 ops). Cortex A57 can handle just 1/4th of that, 4 FMA float64 ops per cycle.
That said, low to mid level servers are not really crunching much numbers. They're all about branchy code such as business logic, encoding / decoding, etc. Or waiting for I/O to complete.
So why would you care about math in a low end server CPU if it's not being used anyways?
This chip is interesting not because of the cpu core in it, but because it has two presumably fast 10GbE interfaces and possibility for a large amount of ram in a cheap-ish chip.