In a normal sentence, you don't include "do" before the verb:

These look good for a mobile phone.

But you can use "do" for emphasis in sentences like the one above. In this case, the speaker says "these do look good" instead of "these look good" because he's emphasizing that he agrees with the speaker. You can also use "do" in this way when you're saying something different from what you or another person just said:

I'm not a big fan of the current prime minister. I do agree with him on defense, though.

In these sentences, you place a strong stress on the word "do".