First off, I understand how reputation works from reading the help center.
Today I ran across this for the first time when I signed up on the musicians' SE site. I was confused as to why I got all this free rep. After looking into it, I see that there is a good rationale behind starting you off right on a new site, as explained by this paragraph:
If you are an experienced Stack Exchange network user with 200 or more reputation on at least one site, you will receive a starting +100 reputation bonus to get you past basic new user restrictions. This will happen automatically on all current Stack Exchange sites where you have an account, and on any other Stack Exchange sites at the time you log in.
That's very clear. It explains how it works. I'm not asking "how" it works, but why. I have read this question as well as several others, and they only explain what it is, and that it is by design. My issue is exactly the same as described in this question, but again, I understand why it happened--it is by design--I just don't understand why it is by design.
Why do I receive this rep bonus on my first SE account?
It makes sense I should get the bonus on a new account, as it says above, to get past some new user restrictions. But if I already have 200 rep on another site (this is the reason I'm getting it on the new site), why give me 100 rep on the old one as well?