The earliest computers didn’t really have motherboards as we conceive of them today. All peripherals and other parts of a computer were not centrally located or connected to a central board, but rather were printed on separate individual boards and connected by hundreds or even thousands wires on a backplane. These boards slid into a vast series of slots contained within a mainframe. Not exactly space efficient.
Eventually in the mid 70s, microprocessor technology advanced and shrunk to the point where more of these functions could be collected on a single printed board—the motherboard. This centralized essential computer functions onto a single piece of circuitry, minimally including the the CPU, main memory, and BIOS, and potentially including keyboard, mouse, audio, video, networking, and others. Those functions that are not located on the motherboard are directly connected to it via a series of sockets on smaller daughtercards (geddit?) This was a fair more economical set up, and one in many ways that enabled computers to launch into the mainstream as being priced for the consumer.
Now, the degree to which peripheral computer functions are located on the motherboard or off has been the primary variable in the development, and in many ways motherboards have moved to both having more and less peripheral functions actually located on it.
The more modular the device, the more useful it is to have peripheral functions located off the motherboard, even though the system is still quite centralized. This particularly applies to laptops and other highly customizable pieces of hardware. Such motherboards typically have lots of sockets that other peripherals may be plugged into.
Devices that aren’t modular, however, such as televisions or cell phones, tend to have all their peripheral functions located on a single motherboard, termed an integrated motherboard. This is by far the cheaper and more space efficient of the two motherboard models, but simply isn’t practical for many devices.
So, peripherals tend to be mostly located on the motherboard, or mostly off, with little in between. Over time, these two opposite trends have been increasing.
Today, motherboards have been largely standardized to fit a few form factors. These change slowly over time, at a far slower rate than other components in a computer, primarily because absolutely everything has to connect to it, making it the most important thing to be compatible with and thus limiting the rate of innovation.
Different motherboard manufacturers use different form factors, which means that every type of device connecting into the motherboard corresponds to different companies. IBM is dominate here, but there are other dissident manufacturers with their own standards as well. What will happen with form factors is anyone’s guess - perhaps standardization with open standards, for the consumer’s convenience if nothing else.
Shrinking It Down
What allowed motherboards to exist was the technology shrinking down to a reasonable size—and it keeps on getting smaller. While there’s certainly a bottom limit to the smallest size, that limit has yet to be found, and with it the limit of computers in general.
A result of shrinking down the size has been an increase in heat output, which leads to the danger of overheating. In turn, there has been significant development in creating efficient heatsinks so that heat will be dissipated in a safe manner. Indeed, it could be that this will be the limiting factor to further decreases in motherboard size.