Mezzanine card slots (?)

Hi, recently I've been looking into blade servers, my old M600 specifically.

What I want to ask here is what are the mezzanine card slots in blade servers? Seen here: http://imgur.com/a/2lMcU

What I've gotten after looking around abit is that they are like expansion slots where I can insert types of cards like network cards to further enhance or compensate for my blade functions?

For example this post answered that they are just an expansion possibility like extra PCI slots: https://www.experts-exchange.com/questions/28179068/Mezzanine-cards-in-Blades.html

So am I actually able to use a blade without inserting any cards in these slots, leaving them empty? 

Also, concerning network interface cards, blades themselves do have these right? So I wouldn't need extra ones in the mezzanine slots if the one on my blade itself is sufficient.

Thanks alot!