The bing api you are talking about does not give the "Today's Homepage" image. For that you have to like they do it, the way you said, i.e. hpimagearchive.aspx thing.
The BingHomepageAPI provides an easy way to do it so that more people can get it without going through the trouble of locating the HPimagearchive page and understand how it works to be able to implement it.
As for why I made it, I have been using the method for approximately 5 months on my website. But the thing is, I didn't find any thing to help me to do so "the easy way", so, to help people in the future I made it open source.
Why use it? Well, you don't have to... If you understand how it works, well do the way you like it. No one, I repeat, no one is forcing you to use it.
http://cdn.muzzammil.xyz/bing/bing.php?format=text&cc=IN
Which is fine if you trust that. But better might have been accessing the API endpoint directly (http://www.bing.com/HPImageArchive.aspx?format=js&idx=0&n=1&...)