"using the same array", what is this same array?

The one referenced by the reference returned by a call to fetchrow_array and the one reference by the reference returned by a earlier call to fetchrow_array.

I don't undertand why perl DBI has created such a seemly unuseful function.

Efficiency. It can avoid creating new arrays all the time, which in turn, allows it to use bind_col internally.

And contrary to your claims, it's not unusable.
If you want just one row at a time, then what's the problem?
If you want all rows, then why aren't you using selectall?