why we need `for( auto& node: tree.nodes)` inside `mbf_behavior_tree.cpp` node file?

[MortezaAliyari]

Hi,
I'm using mbf tutorial package. everything works fine. My issue is for loop inside the mbf_behavior_tree.CPP node file. when I comment the for loop inside the main function the robot skipped moving toward the goals.
why this loop is necessary? and
what is the meaning of conditions inside the loop?
if( auto attempt_next = dynamic_cast<AttemptNext*>( node.get() ))
thanks for your support.