Why do we put punctuation at the end of sentences? When it's over, its over?

The main reason is one would not know a sentence is over without a period or other appropriate punctuation.

Because other wise it is one big mess you see I am that which is it that which it is that thing over there you know any particular thing bears cats hat and fat chicks

In short Christopher Walken English is not considered received pronunciation.

And now the news here on BBC your host Chris Walken is here to tell you the news today he knows what it is and is here to let you know the news here aint he a great guy you know now how about some music from the Kinks here to help ease you into the news here you know

So we know when it's over.

they didn't always this is what it looked like really confusing isn't it it is to tell  readers that one sentence is over if we did not do this it would be very hard to read out loud you'd always be out of breath not a good thing it must make your parents so proud you have turned out to be such a good boy and as we know there are no stpuid questions except when there are do you understand what i am saying