Em inglês, quando podemos utilizar a palavra "seeing"?

Se a estrutura "What do you see?" deve ser usada em vez de "What are you seein?", então, em que situações podemos utilizar a palavra seeing?

Pensei até que ela não fosse usada, até ver em um livro o trecho abaixo:

I sat down and looked at the menu, trying to think about food. But something was worrying me. What was it? I couldn't remember. Something Mr Balfour said that morning? I didn't know, but seeing Derek Halliday reminded me there was something.