I think the answer to your question is that yes, inserting them is for more than just the staff view. It allows SONAR to use it's quantization, groove, audiosnap, and pitch-shifting tools when executing editing functions so you can continue to write/score using them. Otherwise you'd just be left to your ears to manually position things....ugh to that!
:-)
 

Maybe the question should be: why are insert meter change and insert key change together rather than separate operations?
 

Key change also changes how sonar displays note names in Event List, so if you are working in G major it can be disconcerting to see C flats instead of B. 

Key changes aren't as important to me as I'm not interested in how the Staff View looks.
 
But try doing prog rock without meter changes.

As I write this, I am working on a song that is in 4/4; however, I chose to cut one measure in half before each chorus to make it move more urgently.  I inserted a meter of 2/4 for that measure and then went back to 4/4 for the next measure.  If I had not done this, all the verses and choruses after that point would have started in the middle of measures which would be confusing.
 
I also use key changes so the staff view does not look like a mess, and so Sonar will help me by inserting notes that are in the scale as the default.
 
Naturally, if you are doing a waltz (3/4) and just accept Sonar's 4/4 default, every bar (measure) will be off.  It is a lot easier to think of a chorus as starting at bar 20, for example, than at bar 13.5 or whatever.