In the embedded world, you are limited by the amount of storage space available for your code. In a lot of cases the code is in flash or one time programmable r

In the embedded world, you are limited by the amount of storage space available for your code. In a lot of cases the code is in flash or one time programmable rom memory on the microcontroller. Even if you have off-chip storage, you usually have to load the code into ram in order to run, and ram size is also quite limited. For low end microcontrollers it's common for rom / ram sizes to be on the order of 32kb (although they go much lower than that). In this world, the most common optimization level is -Os (or -Oz if available) because running out of storage space means you need a more expensive microcontroller, which increases the cost of each device you are selling, and might require a redesign of the PCB.