HTML attribute vs Dom property

I've read some posts on that issue and still couldn't understand. I understand that html attributes are initial values what is rendered when executing our html code. then we change dom elements properties. so If it's that way, why do i need to have that information about my initial values? if i have this input element, after rendering the page, as i understand it's a simple object which has some "variables" and "methods" , and what's the difference between that and dom object?