In order to express your curiosity as to "WHAT" is good, you can ask:

? [mwo-GA jo-a-yo?]

Here the word 가 [ga] emphasize what the subject of the verb "to be / 좋아요" is.

If one says,

ABC . [ABC jo-a-yo] = ABC is good.

And if you disagree and you think XYZ is good, not ABC. You can express your opinion that the subject of "being good" should be XYZ, not ABC, by saying:

ABC ? XYZ가 ! [ABC jo-a-yo? XYZ-GA jo-a-yo]

So, with 이 and 가, you can add more flavor and more concrete meanings to your Korean sen-

tences when you want to emphasize WHO did something, WHICH ONE is good, and etc.

Almost.

As you can see from above, 은/는 and 이/가 both have different roles, but the roles of 은/는 as a 'contrast' factor is much stronger, because you can change the topic of a sentence with 은/는.

So, when you form complex sentences (i.e. I think the book that you bought is more interesting than the book I bought.), in general, 은/는 is not so commonly used all over the sentences. Often times 은/는/이/가 can be dropped, but when you need particles to clarify the meaning, 이/가 will be more commonly used.