a result set in one array. This is handy if want to fetch all results and
pass the data directly to a template engine, for example. In this case
you save a loop.
1) Currently:
$all_rows = array();
while ($row = mysqli_fetch_assoc($res))
$all_rows[] = $row;
2) With mysqli_fetch_all:
$all_rows = mysqli_fetch_all($res, MYSQL_ASSOC)
Note: if you need the loop for processing each row, try variant 1) for
performance reasons.