Mobile phones run on so-called embedded chipsets, which are designed to perform one or a few dedicated functions, often with real-time computing constraints. They are embedded as part of the complete device including hardware and mechanical parts.
The ever popular smartphones are equipped with more advanced embedded chipsets that can do many different tasks depending on their programming.
Thus their CPU (Central Processing Unit) performance is vital for the daily user experience and people tend to use the clock rate of the main CPU that's in the heart of the chipset to compare the performance of competing end products.
As we already pointed out, the clock rate of a processor is only useful for providing performance comparisons between computer chips in the same processor family and generation.
Also, as mobile gaming is increasingly gaining popularity, users have become more aware of the various types of GPU (Graphics Processing Unit) chips that come as part of the mobile chipsets and sometimes even consider their performance when making buying decisions.