Modal verb "will" to describe a present situation

Question

I have heard a phone conversation between a person wishing to buy theater tickets and a sales agent like the following:

“I want to buy two tickets for tonight’s show.” ~ “Certainly. I’ll need your credit card information.”

I believe “I need your credit card information” makes perfect sense. What are the purposes/effects of the modal verb “will” here?

Another scenario: let’s suppose the theatergoer asked the agent something for which the agent has no ready answer. In such a case, I know one would say something like,

“I’ll have to get back to you on that one.”

Again, I believe the sentence is fine without the “will.“ So, what nuance does a “will” produce here?

Answer 1

You're absolutely correct: "I need" is logically sound. However, "I'll need" sounds more polite.

Although the other answers posted here state that "I'll need" is more correct because the clerk really needs the credit card information in a few seconds (presumably when he enters in the number), technically it sounds like he needs the card right there and then, so I don't believe that's quite the right answer.

So why does "I'll need" sound more polite? Saying "I need your credit card" sounds like a demand, akin to "Give me your credit card." Throwing in extra words, like conditionals ("Would you pass me the salt?" vs. "Pass me the salt"), tends to sound more polite, even if the sentence without them is technically correct. It somehow softens the blow of something sensitive like asking for someone's money.

EDIT: I realized that this doesn't address the other question: why it's "I'll have to get back to you on that" vs. "I have to get back to you on that." Again, I agree that the latter is technically correct. Right now, I have to get back to you on something, even if I end up getting back to you at some point in the future, like saying "I have to get some milk." The thing is, that particular phrase is an idiom, and there's really no explaining idioms. It's just how it developed and how we're used to hearing it, and you better say it that way or it'll sound off to everyone.

Answer 2

The moment you use 'will', it takes you a bit further in time though the thing is happening after some time.

Imagine a conversation between my secretary and me. I'm quite sincere about my work.

Okay. Tina, you need to make a call to Robert. We need to make the deal final by evening.

Tina replies...

Okay sir, I'll do that.

I reply...

"will?...do that now".

If you see, removing 'will' shows that the event happens 'right now'. However, we often consider I call you in 5 minutes and I'll call you in 5 minutes similar.

Coming to your sentence...

I'll need your credit card information

talks about the agent booking the tickets in some time. I'll call it an 'early future' if not distant.

This is because 'future' is a bit broad word. It ranges from the next moment to the time infinite.

What happens next, no one can say (while watching a mystery)

What will happen next, no one can say (predicting the future after attacking on a terrorists' country. The attack could have been planned 5 years from now!)

Answer 3

In your first example applies the definition #2 from American Heritage of English Dictionary of will used as an auxiliary to indicate likelihood or certainty:

Certainly. I’ll need your credit card

In your second example applies the definition #5 used to indicate intention (used as auxiliary as well):

I'll have to get back to you on that one.