Well I'm a Front-End developer and I've been asked for my portfolio and I can tell that what they like to see is big companies in it or big and cool projects that they know about. I think what they are looking for is to see if they've seen your work before without knowing or see that big companies (McDonalds, Coca Cola, Pepsi.. for example) trusted you for doing some design or development.

I've never been asked for a portfolio, maybe if you're a frontend developer focussing on design, but never for frontend developer or backend developer roles have I been asked for a portfolio. Don't know if it's different in other parts of the world ...