Do you give a two week notice or just walk out?

I've always wondered why giving an employer a 2 week notice became a thing. That same employer more often than not would lay you off with no notice whatsoever. I get the whole not burning bridges thing but why is it wrong to leave for more money without notice when the same employer would cut you without notice to save money? In the industry I work in more often than not an employer will send you packing when you give your notice. People still do it, though.

I've never walked out on a job but it I got to thinking about this over my morning coffee. Do you baws give a notice?