Developers of desktop applications usually pay little attention to the hardware architecture. First, they often use high-level programming languages that hide these complexities at the cost of some performance drop. Second, in most cases, their code runs on x86 architecture and they often take its features for granted. For example, they may assume that the size of int is 32 bits, which is not true in many cases.
Embedded developers deal with a much wider variety of architectures. Even if they do not write code in assembly language native to the target platform, they should be aware that all C and C++ fundamental types are architecture-dependent; the standard only guarantees that int is at least 16 bits. They should also know the traits of particular architectures, such as endianness and alignment, and take into account that operations with floating point or 64-bit numbers, which are relatively cheap on x86 architecture, may be much more expensive on other architectures.