Why alignment is 16 bytes on 64 bit architecture?

gcc is not "assigning" this space to the variable. Rather, the x86_64 abi requires the stack pointer to always be 16-byte-aligned at function calls, in case the callee uses vectorized SSE math. It's a really stupid and wasteful requirement (the callee should ensure the alignment if it needs it), but that's the standard, and gcc follows the standard. You can fix it with -mpreferred-stack-boundary=3 (8 byte alignment, the minimum for 64-bit).

It is 8 bytes, not 16. The LEA instruction doesn't show anything alignment related, -0x10 is just an offset applied to the value of the RBP register. Probably to generate the address of a small local array. If the code generator uses any SIMD instructions then 16 could be relevant. None of which is visible in a two-liner question.