The reason why it will help him is that he was disciplined and on point. To many wavering voters Trump comes across as an inept reality TV entertainer (including yours truly
). In fact, Clinton's main premise has been that Trump is not up to the task and simply a huge risk.
Tonight he came across as a slightly smaller risk, while hitting every downtrodden constituency for their vote. It will also help by making people forget Melania's plagiarism, Cruz's non-endorsement and other fun things from the convention