I don't like to condemn new usages, and there's little point ranting about pet peeves, BUT... does anyone else hate the pointless use of go ahead and... ?

It seems to occur most often in unscripted speech (often in instructional videos), and so far, I've only heard Americans use it.

The phrase "what I'm going to do is, I'm going to go ahead and ..." gets over two million hits on Google. It contains twelve words, and can be replaced with three: "I'm going to ...", without losing any meaning, explicit or subtle, as far as I can tell.

Adding now after do makes it even more pointless; Google gives only five hits for that phrase.

Can anyone suggest why some people use go ahead and ... ? Do they think it sounds more professional? More polite? Or do they say it just because they've never heard of Strunk and White or Ernest Gowers or Richard Mitchell?

Please also feel free to tell me not to waste your time with an exercise in futility :-)
Now, that is a surprising rant. I'm sure I must say 'go ahead and...' reasonably frequently with no second thought. It is informal, of course, so no question of formality or politeness. I think there are two basic uses:

1. I'm going to go ahead (now) and replace this widget. Here it is an expression of determination or decision, often before a final consensus or conclusion about the intended action is reached.

2. If you don't like my cooking, then you can just go ahead and eat out from now on! Here it challenges the listener to make that decision (in the heat of anger, of course).

On the other hand, preceding it with 'what I'm going to do is' seems to me just a common communicative filler (it has a name that has slipped my mind) used to introduce or draw attention to the utterance that follows:

What I'm going to do is screw this plank to the bulkhead and....
What he's going to do is, he's going to wait for us at the station, then...
Style guides are only appropriate (if they are appropriate at all) to certain types or writing. If you follow them when speaking your speech is likely to sound very stilted.

Strunk and White's guide is now nearly a hundred years old and was always of dubious benefit.

Gower's The Complete Plain Words is a lot better, but was aimed at civil servants. He quotes Dr Johnson:

"Academies have been instituted to guard the avenues of the languages, to retain fugitives and to repulse invaders; but their vigilance and activity have been vain; sounds are too volatile and subtile for legal restraints, to enchain syllables and to lash the wind are equally the undertakings of pride, unwilling to measure its desires by its strength".

Orwell famously set out rules for clear writing but the last was the most important:

"Break any of these rules sooner than say anything outright barbarous."

Speech is different from writing and tends to use words and phrases which do not add strictly to meaning but which nevertheless often serve a purpose. Many of us home in somewhat selectively on apparently superfluous phrases and develop an irrational dislike of them. The most severely affected write letters to the BBC and The Times about it.
Students: Are you brave enough to let our tutors analyse your pronunciation?
I'm an American and use this phrase without thinking as well. It's probably frequently heard in business, but the idea is that you are in some process, so when you complete one step, you "go ahead (to the next step)" and do something else. People definitely say with the sense of moving on, progressing to something they had been wanting to do, but were unable to because of the steps before it.

The peculiar charm of "Why don't you go ahead and shut up?" comes from this sense of progress to the meaty step in a process.

I share the same aggravation for the people as the prior. Also, working in the food industry, find many people asking if they can "do" a product that they want; "Can I DO a small caramel latte" or "id like to DO a large iced coffee". Do you not remember the word HAVE or WANT?! I know you DO so USE them!