Melania Trump represents a softer, kinder image for the Trump campaign. She can help her husband campaign and bring in more female voters, especially married women who usually vote Republican. Melania can also advise her husband on behaving more presidential on the campaign trail; improving his image with the voting public.
Yes, generally everywhere in the world people subconsciously (or intentionally) vote for those candidates who are more beautiful. As we can see here, Hilary and Donald are not the same genre, so the focus falls onto Melanie to fight for her husband's victory with her beauty. Believe it or not, many voters participate in the elections subconsciously.
Melania Trump may be a beautiful ex-model, but outward appearances are not always everything in such a demanding job as First Lady - or even potential First Lady. Melania Trump will not be able to court voters because more people are seeing just what a terrible candidate Trump is and are therefore deciding not to vote for him.
Donald Trump's wife Melani did look great at the debates last night. But she is in no position to even boost a point for Trump. She has been fairly quiet on the campaign trail, and is most noted for her deplorable stolen speech at the RNC. If we compare her to our current First Lady, she can barely hold a candle. I bet if she came out with as much energy and original content as other First Lady contenders of previous elections, she might have a chance at helping Trump, but sitting and looking pretty does not sway many voters.