A system on module takes the hardest part of a hardware design, the high-speed layout around a microprocessor with its DRAM, flash and power rails, and delivers it as a tested board the size of a credit card or smaller. The product team designs only a carrier board with the connectors, sensors and power input their application needs, and the module plugs in through an edge connector or board-to-board headers.
The trade-off is unit cost against engineering effort. A SoM costs more per piece than the bare chips, but it saves months of layout, bring-up and electromagnetic compliance work. For volumes in the thousands rather than millions that is almost always the better deal. The Raspberry Pi Compute Module is the familiar example; industrial vendors offer SoMs with long availability guarantees.
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