[Elecraft] KPA500 question

[Slava Baytalskiy]

Hello everyone!
I'm a proud owner of a new K3/P3.
Almost there...
Can someone please explain to me the rationale behind the KPA500? Why 500W?
Is there a tremendous difference between legal limit and about half of it (which is what i hear KPA500 can do)?
I understand the ease of use, how the amp follows the radio. I get all that. But why half of max power?
Chances are i won't be buying any more amps for the foreseeable future if i get the KPA500 at Dayton this year, so i'd like to know that it will cover everything i need it to (and i have no idea what that might be).

I hope this question isn't too confusing.
I'm a new ham (almost a year new), so please be gentle.
Thanks in advance!
__________________
Slava (Sal) B, W2RMS
W2RMS at arrl.net