How to add a third-party repo. and key in Debian?

I looked at another similar question about adding third-party repos. I am trying to add a third-party desktop IM client called riot . While the site gives link to the third-party it gives no instructions as how to add third-party sources or keyring in Debian. I went through https://riot.im/packages/debian/pool/main/ and made the following additions in my /etc/apt/sources.list –

What is the benefit of /etc/apt/sources.list.d over /etc/apt/sources.list

I know this question has been asked before, but I do not accept the answer, “you can clearly see custom additions”. When I add ppa’s (which I have not done in years), I hit a key on my keyboard labeled “Enter” which allows me to add an empty line before the new entry (I would even add an explanatory comment, but I am a tech writer, so ….). I like my sources.conf clean and neat.