Question aout email, icedove/thunderbird

[Paul E Condon <pecondon@mesanetworks.net>]

I'm learning to use icedove after a long and happy relationship with 
mutt. I remember having this question from before I knew about mutt and 
it still puzzles me: Why the checkbox labeled "Always check to see if 
icedove is the default mail client on startup"

How does icedove/thunderbird behavior differ if it finds it is NOT the 
default or it finds is IS the default? Is there some reason why it 
matters to me which way I have this, checked or unchecked? I can't find
any explanation of what this is, other than the tautological "Tell 
icedove if it is you default mail client." Where have I set my default 
main client?

TIA

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