This has been happening to me so many times in my life, but three times within a week has let me fuming.
Buying some fabric and told the lady how much I wanted so she could cut it.
Salesperson: ”What do you need it for?”
Me: ”I only need one and a half metres, please.”
I needed a sheet of plywood cut to size, and made a drawing of the cuts, including all the information needed, type, thickness, measurements.
The salesperson looked at it and said: Well, that's pretty clear. What do you need it for?”
Me, dodging the question: ”Can you cut it now, or do I have to come back tomorrow?”
Then I wanted to buy a certain paint that I have used before. It was nowhere to be found in my town, but the net told me that a store in a nearby town had it, so I drove there. I told the salesperson the make, colour, gloss, and amount. All he needed to know.
Salesperson: ”Never heard of it. I'm sure we don't have it.”
Me: ”Your website claims you have six buckets in stock.”
He checked his computer, didn't say anything, but then walked around the store for ten minutes, looking for it. He finally found it, came back to me, and I payed.
Then the question came: ”What do you need it for?”
I swallowed down a snide remark and just said: ”I am going to put it on my wall.”
When I tell them exactly what I want, why do they ask this question, even when the answer is logical? I was once asked when I bought som towels. Towels, for heaven's sake!

It annoys me sooooo much. It is none of their business when I don't ask for advice.
I actually feel a little better now.
Thanks for reading.