Anyone who either doesn't have the time, talent or money to create their own content or outsource it to a professional ghostwriter can benefit from using private label rights.
Even if you don't alter it, you can plug it in as is in an autoresponder system or compile it to make an eBook out of it and have an instant stream of communication with your subscribers or a readymade product for sale.
You can use the PLR in any fashion EXCEPT to sell them AS private label rights yourself (that's MY job) 
Yes, whether it's an eBook, article, blog post, or autoresponder - you can slap your name on it as the author.
You don't HAVE to, but it's a good idea. Generally, I recommend that you alter the title, and then either add or alter the first and last paragraph. Some customers like to swap out keywords as well or reorder the article.
No, in order to keep the costs this low, I do not place limits on the number of packs sold. However, if you're altering your PLR, this won't be a concern at all.