Recently, I was asked like why do we have a web application? What's the use of it? After listening to my answer I was questioned on the same as why can't we do the same for a site collection directly? Instead of creating a web application we can create a site collection itself with the same settings and all. At last, I convinced the questioner that there would be performance and security issues.
But I want to know the exact reason why it can't be. I would be grateful if anyone can help me get a better answer.