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Question 1: In the The main reference on algebraic stacks (Laumon and Moret--Bailly) defines a separable algebraic stack as one having universally closed diagonal. For schemes it separability is simply defined by the condition that the diagonal is a closed immersion. Why this difference?

Question 2: Presentations of algebraic stacks are defined (in LMB) as morphisms (with some properties) $X\to\mathscr{X}$ with $X$ an algebraic space. What does one lose by only considering schemes $X$ instead?

These questions are certainly well-known to experts in stack theory and I know there are a few around on MO, so I'm hopeful that I will be sufficiently enligtenedenlightened.
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Two questions on algebraic stacks

Question 1: In the main reference on algebraic stacks (Laumon and Moret--Bailly) defines a separable algebraic stack as one having universally closed diagonal. For schemes it is simply defined by the condition that the diagonal is a closed immersion. Why this difference?

Question 2: Presentations of algebraic stacks are defined (in LMB) as morphisms (with some properties) $X\to\mathscr{X}$ with $X$ an algebraic space. What does one lose by only considering schemes $X$ instead?

These questions are certainly well-known to experts in stack theory and I know there are a few around on MO, so I'm hopeful that I will be sufficiently enligtened.