The main attributes of a vice president are to balance the ticket, help raise money and then step back for four/eight years until it is her turn to run. You want someone good, but not too good, especially if you have an ego the size of Trump's.
And with Hillary Clinton as the likely Democratic nominee, a woman VP on the Republican ticket would be no small advantage. Ernst is the first woman to represent Iowa in the United States Congress and the first female veteran to serve -- from any state -- in the Senate. Those kinds of things will pull a lot of the air out of Hillary's calls to empower women in politics.
The Washington Post calls Joni Ernst, currently the Republican senator from Iowa, "a breakout star of the 2014 midterm election with her plain-spoken populism." That matches some of Trump's appeal, and plays well to the populism that fuels the Sanders campaign.
Ernst has been critical of Trump's comments about women, which could be a smart way to make Trump appear more open-minded, and pull in additional female votes. He could also use Ernst's criticism to soften his comments going forward, explaining she has helped him better see the error of his ways.
Ernst also aligns well with Trump and Republican positions.
She's proposed eliminating the Internal Revenue Service, the Department of Education, and the Environmental Protection Agency to cut federal spending. She wants to eliminate the Department of Education "not just because it would save taxpayer dollars, but because I do believe our children are better educated when it's coming from the state." She is also no friend of Planned Parenthood, another signature Republican issue. In 2014, Ernst delivered the official GOP rebuttal to President Obama's State of the Union Address.
And a bonus -- Ernst is a good friend of the Koch Brothers, and her being on the ticket could help them feel better about a President Trump. A lot of Koch money flowed into Ernst's Senate campaign via the Koch-backed consulting firm Aegis Strategic. The brothers are known to be wary of Trump, and having one of their own people in the White House would go a long way to garnering their support.
Keep an eye on Joni Ernst. She might make for some very smart politics by a guy who says he knows a good deal when he sees one.