Reason for separating classes into different divs in html

I have downloaded a Wordpress theme and is modifying the code to make it look more like I want. On many places I come across things like this:

<div class="class-1">
<div class="class-2"> 
<!-- Content here -->
</div>
</div>

I know that you can write them together like this to save both on bytes and (for me) readability, especially since there can be 3-4 layers of these on many many places:

<div class="class-1 class-2"> 
<!-- Content here -->
</div>

1. Is there any reason to keep the divs separated regarding performance, compatibility, securety or other?
2. Am I risking something when lumping them together, or is the code intepreted the same way by the compiler?