What, exactly, is the point of beginning a sentence with "Well..."?

Sample conversation:

Person 1: What did you think of the movie?
Person 2: Well, the acting was great, but the plot was terrible.

What does "well" actually add to the body of the sentence? I have never heard any sentence spoken or written whose meaning was absolutely dependent on beginning with "well".

It seems akin to an "um" or "uh", serving as a vocal void filler while the speaker collects their thoughts and puts their words together before actually speaking them.

If that's the case, why do people actually type it? People clearly have time to think about what they're going to say before posting a comment on a blog or replying to an email. Has it become a sort of accepted language anomaly much in the same way Midwesterners like to say things like "Where did you put it at?"

"Well" does not serve any grammatical usage in this sentence. That's because, in this context, "well" is an interjection, a word that expresses emotion. Depending on how it is enunciated, "well" could indicate impatience, surprise, nervousness, and a variety of other emotions.

However, here it seems to function as a filler, similar to "uh." It doesn't have any true meaning. People type it for the same reason as they type the interjections "oh" or "uh" - to convey emotion or to fill space while thinking of what to say.

1) A pause while the speaker considers a politic answer.

Q: "What did you think of my sister's cooking?" A: "Well ... it was very filling."

2) An indication that the rest of the answer is going to be a mix of positive and negative. See your own example.

There are 8 parts of speech. People seem to have forgotten about Interjections. They express emotion, good or bad. They are the "Golly Gee" and "Holy Cow" from days gone by, which have now been replaced by vulgar slang and swears: We currently see a lot of 'F-ing hell', tons of 'what the f-', the forever loving 'sh--' exclamation.

Well, when used as an interjection or an exclamation is not the same as 'uh, um, er, you know, or like. Those are merely fillers because of stalling and trying to prevent silence. They are not exclamations nor interjections of any emotion.

Well, can also stand for "Well, in my opinion, the acting was good, but the plot was terrible.'

I miss the fact that people no longer exclaim, "Alas!" Ah, those were the well spoken days!

'well' at the beginning of a spoken statement ( answers Or Questions) simply means that the respondent either have a LONG but well planned / well thought point wise answer to the question asked OR the question posed to the respondent is such that requires a recollection of memories i.e. thinking & then replying ( but short answer).