Explaining the "Make a Capital Gain Election" Option

Hello!  For the past couple of years, I've had approximately $10,000 in investment interest expense (from buying stocks on margin) each year.  I never "did anything with this," and each year it carried over. This year I have an additional $11,000.

 

In TurboTax, it is asking me if I want to "Make a Capital Gain Election" -- "Your return shows qualified dividends or capital gains income of $7,000.  You may increase your investment interest deduction by electing to treat as much as $7,000 of your capital gain income as investment income. "

 

Could someone explain this to me?  It seems advantageous to take advantage of this -- when I plug in $7000 for Capital Gains election amount, my refund increases by a little over $1,200.  But I want to understand what is going on here.  Could someone dumb this down for me (LOL)?  I really appreciate it!  Thanks!