There aren't many beneficial reasons for undoing net neutrality, at least for consumers and most businesses, but here are some of the arguments I've seen, many pulled from Ajit Pai's comments.
Some have argued that repeal restores internet freedom, of course, that's doublespeak, as it only restores freedom for ISPs. Kinda like how repealing the Bill of Rights would result in more freedom. This is also the same as the regulation argument: no regulation == good. Which, of course, is false. Some regulation is good.
Other arguments state that the internet was just fine before net neutrality. This is not factually accurate, as there were already moves by Comcast to deprioritize Netflix traffic. Eventually, Netflix had to pay Comcast to be reprioritized. Also, the internet isn't some static entity. It's use is always expanding. Video has exploded as has games, etc. As more of these additional uses are brought to market, especially high bandwidth ones, there will be pressure from the ISP's to restrict their traffic further.
This is also similar to the argument that the lack of previous government regulation is what allowed the internet to grow. While that may be true or not, it doesn't mean that net neutrality is a bad thing. In fact, by creating an open playing field, it should actually foster more competition, which usually helps with growth.
Pai also argues that net neutrality limits investments in networks by ISPs. This also isn't factual. There is evidence that it actually increased for some ISPs.
Pai says that smaller ISPs can't deal with the side effects of net neutrality, thus innovation is stifled. This is a narrow view. Let's assume he is correct, even though he didn't provide hard evidence. Innovation occurs beyond just ISPs. There are lots of companies and products that operate over the internet and net neutrality allows them equal access to all consumers. The innovation that brings far outweighs "lost innovation" from ISPs.
EDIT: Let me add a few additional arguments: One concern is that limiting the ability to control traffic over your network may have serious consequences. For example, an ISP may need to restrict bit torrent traffic, as it is responsible for a growing amount of bandwidth. It's possible traffic like this could reduce people's throughput, the throughput they are paying for from their ISPs. It's seems reasonable that an ISP could slow this down, at least until their infrastructure can handle it. Comcast, before NN, actually began blocking bit torrent traffic. Regardless, there was a provision for reasonable network management in the net neutrality rules, so this argument isn't really valid.
Pai said he wants an level playing field, and cites AMP and promoted tweets as examples where companies don't play fair. AMP, as far as I can tell is merely a distribution format and promoted tweets is how Twitter advertises. What Pai is actually saying is quite interesting. He is in fact saying that ISP's should be able to prioritize traffic based on who pays them, just as Twitter does with it's promoted tweets. This argument gets to the heart of the net neutrality debate. Pai and others feel ISPs should be allowed to prioritize traffic.