Page "Central processing unit" Paragraph 33
Higher levels of integer range require more structures to deal with the additional digits, and therefore more complexity, size, power usage, and general expense.
It is not at all uncommon, therefore, to see 4-or 8-bit microcontrollers used in modern applications, even though CPUs with much higher range ( such as 16, 32, 64, even 128-bit ) are available.
The simpler microcontrollers are usually cheaper, use less power, and therefore generate less heat, all of which can be major design considerations for electronic devices.
However, in higher-end applications, the benefits afforded by the extra range ( most often the additional address space ) are more significant and often affect design choices.
To gain some of the advantages afforded by both lower and higher bit lengths, many CPUs are designed with different bit widths for different portions of the device.
For example, the IBM System / 370 used a CPU that was primarily 32 bit, but it used 128-bit precision inside its floating point units to facilitate greater accuracy and range in floating point numbers.
Many later CPU designs use similar mixed bit width, especially when the processor is meant for general-purpose usage where a reasonable balance of integer and floating point capability is required.