Casual, smart casual, semi-formal, formal, black tie, white tie--these are some dress codes you see on invitations to certain occasions. Knowing the dress code can spell a huge difference. For the host, it lets him/her know that you put thought into what to wear for the occasion, meaning you respect the host enough to dress appropriately for the event. Can you imagine having someone come in smart casual clothing for a FORMAL wedding cause they thought it was semi-formal (and semi-formal = almost formal anyway)? If it were my wedding, I wouldn't be mildly insulted; I'd be flat out insulted.
While a lot of people disregard dress codes, I do know some that want to follow through but get confused about these kinds of things--like aren't smart casual and semi-formal almost the same thing? Well, I am here to help you out.