Why do we have silent letters in the English language?

There is a particular word (which I forgot) but its origin is of laziness and practicality. People in the olden days used to say certain words so fast (due to business relations stress, certainly in the Industrial Revolution) that they started omitting consonants.. and then it became accepted matter.

I think an example might be that of 'actor'. In other languages it used to be 'actor' and now it became 'ator', which old language purists look down against.

Language varies from time to time; it is ever-changing.