What is the purpose a 1,0 or 0,1 loop at the end of the language DFA?

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Here is a finite-state automata without output:

Language recognition automata without output

In finite-state machine with output, I understand that the 1,0 or 0,1 would give an output of 0 if input is 1 and the opposite.

However, what I don't understand is that in this finite-state automata without an output, what purpose does the 1,0 serve?

Why don't they just write a 0 on the loop instead?

What would happen if the input at s1 is 1 if there is no where to go from the state if input is 1?