You can create a new empty array of variable size backed by a 32 Byte array as it's starting size.

The difference is really that [32]Byte is a single pointer (in compiler hands that you never touch) to a slab of 32 bytes of memory; the []Byte (of internal size 32 use 0-32) has the current allocated size, current used size, and a pointer (none of which you directly touch but two of which are trivial to affect with language semantics) values that point to a 32 Byte backing array.

The only time this matter is timing or performance critical code. With respect to cryptography, timing might be critical for performance, but it's absolutely critical for never taking _variable_ time to perform an operation based on data as well as not on key. In that respect this doesn't matter.