This is not simplification. It is canonicalization. Bringing expressions to canonical form is very useful because then two expressions can be compared for equality by simply checking that they have the same structure. Checking equality is a very common operation during symbolic manipulations.
b+a are structurally different, but mathematically equivalent. Sorting brings them to the same (canonical) form:
a+b. Now they can be trivially compared.
Some limited canonicalization is done automatically by Mathematica. This is what you see here.
Canonicalization does not optimize expressions for readability by humans. It optimizes them for computation. For readability, use
TraditionalForm. It will display
b-a (even though the underlying structure is still
TraditionalForm is not smart enough to handle the example you show, i.e.
-Sin[a-b]. I am not aware of any existing formatter that handles this the way you want. You can attempt writing one using replacement rules, but I think covering all similar cases is almost hopeless and not worth the effort ... there are too many possibilities.
You can prevent any evaluation/canonicalization using
HoldForm[Sin[b-a]], but this just maintains the expression in its original form. It is not a means to convert a computation result back to the form you ask for.