수가 is for emphasis verb. In English, it is similar to do.
For exmaple, "I agree with you", "I do agree with you".
Then, "너 때문에 살 수 없다.", "너 때문에 살 수가 없다.".
But, it is a subtle nuance between "수" and "수가". You would use them both anytime.
I hope it helps you.