What is the purpose of having randomizer cards for the non-kingdom cards?
They are there to mark each pile.  The game ends when 3 piles are empty.  Once you see 3 randomizer cards showing, the game is over.

This is on page 5 of the rules:


  Players may also use the Randomizer cards as Placeholders to mark the card piles so empty piles are easily seen


The way my group lays out the Kingdom cards is organized by cost, so it is not always obvious that there is a pile missing.  The kingdom cards help remind us.

I agree with your comment that the Randomizer cards are a bit too similar to the normal cards, one solution is to put them into different colored sleeves than your regular cards.  Or perhaps, just sleeve the randomizer cards.