In C++ block is often a way to ensure automatic variable destruction. This is important, because in C++ often there is important stuff going on in destructor. For example in RAII idiom.

Example:

   ...
   ...
   
   { //critical section
     QMutexLocker locksInConstructorAndUnlocksInDestructor(&mutex);
     a = doStuff(a,b,c);
     b = doOtherStuff(a,b,c);
     c = andAnother(a,b,c);

   }
   
   ..
   .. 

It would be overkill to make this block a function, especially when code in critical section changes many variables.