On the expression "some... and not others"

I want to ask a simple question.

One often uses some and and not others or but not others together. For instance:

(1) Why does cancer attack some tissues but not others?
(2) Why do interventions work in some places and not others?

Is and not others or but not others put after some just to emphasize the noun after some?

Yes, for emphasis and also to avoid ambiguity: "Some" does not necessarily mean "some but not all". It can mean "at least one, maybe all". So adding "and not others" ensures that you don't mean it that way.

Some usually represents an Existential Quantifier. That's one of two types of logical quantifiers (the other is the Universal Quantifier, usually represented in English by each, every, and all).

The existential quantifier asserts the existence of (for count nouns) at least one object, or (for mass nouns) at least some minimum quantity -- and maybe more in both cases -- of whatever argument it binds. The universal quantifier refers to all of whatever it binds.

They are related in the sense that if something is true for All X, then it's clearly true for Some X; while if all we know is that it's true of Some X, we can conclude nothing about its truth for All X. If all Hoosiers were left-handed, then certainly some Hoosiers would be. But all we know is that some Hoosiers are left-handed, so we just don't know about all Hoosiers.

This is logic -- not, strictly speaking, grammar -- but English, like all languages, does have quantifiers as a normal part of speech, and the meanings of these particular quantifiers are modelled fairly well by Quantified Predicate Calculus.