To ensure that member is itself still aligned properly in "global space". The start of the struct is assumed to be universally aligned (malloc, etc.. make that a requirement in fact) or aligned for the requirements of the struct itself (eg, array). Thus any offset into the struct only needs to be aligned to the requirements of the largest type.

https://www.kernel.org/doc/html/latest/core-api/unaligned-me... has a lot more general context on alignment and why it's important