I wouldn't say this is syntactic sugar here. packed structs are the most valuable tool in the language to handle interop with other languages and building proto

I wouldn't say this is syntactic sugar here. packed structs are the most valuable tool in the language to handle interop with other languages and building protocols. The fact that you can use that tool to build bitfields feels like an interesting, cool and useful side effect. However, it's not their purpose. Note that the main difference between packed structs and regular structs is not the dense bit packing, rather that regular structs are allowed to reorder the fields however they wish in memory. The compiler is free to optimize your struct. It's not free to do so in other languages (like C for example). Thus you get a way to define structs exactly, with bit level precision, to build complex protocols where you can decide what every bit means.