The [Ins] key is used as a generic repetition tool for repeating the last action. It works nicely for wires and labels (with an auto increment function too), but it does not work very well to repeat adding a symbol to the schematic. I think this is a bug, and there is some more in the following thread: Schematic grid snapping bug or feature? - #4 by paulvdh
I forgot about that “Place repeated copies” button.
I tried it a bit, and quite quickly disabled it again because now you ave to use [Esc] or Cancel twice each time after adding a symbol. And when you average that out, then this mostly counteracts the usage of that checkbox in my point of view. Repeating symbols can be a nice feature, but if you have to toggle checkboxes too often then the overall result is too small to bother with.
For bigger arrays, the Duplicate that Piotr suggests is more convenient and faster.
But apparently you have a different opinion (use case), and that’s fine. That’s why the checkbox is there, so you can also do it your way