Some silent letters were put into English because the first printers were not English speakers and they were making things up. The h in 'when' and the h in 'ghost', for example.

Jack, it's a good question where the l in 'could' came from.

The k in 'knight' is an example of what Jack's talking about. I'm pretty sure the word used to be pronounced ka-nicht.

Meanwhile, I believe that if I were singing 'Couldst Thou but Know' by Michael Balfe, I would folk process it to 'Could You but Know.'