A sync.Pool is used to avoid unnecessary allocations and reduce the
amount of work the garbage collector has to do.  When passing a value
that is not a pointer to a function that accepts an interface, the value
needs to be placed on the heap, which means an additional allocation.
Slices are a common thing to put in sync.Pools, and they’re structs with
3 fields (length, capacity, and a pointer to an array). In order to
avoid the extra allocation, one should store a pointer to the slice
instead.