In real life, when a person drinks any drink from a straw, you would only hear a noise when his/her drink is about to finish.
I mean this noise:
But in TV shows, whenever you see someone drinking from a straw, it never matters how empty or full the drink is. That sound will always be heard.
Is it done deliberately (sound effect is added later) so that audience can know for sure that the person is really drinking (as opposed to : just putting the straw inside the mouth and not sucking anything)?
This irks me too, but I'm certain it's done to convey to the audience that the character is actually drinking. It's one of those tropes that people have become accustomed to, even though it isn't realistic. It's similar to the trope of "every time you turn on a microphone, it whines with feedback for a second, then settles down." Not realistic, but it clearly shows the audience what's going on.
Is it done deliberately (sound effect is added later) so that audience can know for sure that the person is really drinking (as opposed to : just putting the straw inside the mouth and not sucking anything)?
Yes. Most movie sounds aren't designed to be accurate so much as they are designed to convey to the audience what's happening. We could argue that since a movie watching audience's attention isn't always 100% focused on every aspect of the screen instead honed in on the major parts like actor's faces and such that these small sounds are used to make sure the audience follows along with minor actions that add up to complete the mise en scene (if I'm using that correctly).
You can also look at things like walking up stairs. The sounds made are usually artificial. They creak when the filmmakers need to create tension, and are silent when the person is effectively sneaky, and give just the right amount of thud otherwise, each and every step sounding exactly the same.
