"> 1) the stuff is not available on the internet
Actually, it probably is."
It may be, but (especially if you're not an expert in that domain), finding it can be tricky if you don't know the actual name for it. I've had that a few times and it can be easier to just nip down to the shop and look in the most likely aisle/ask a staff member, rather than hunting on Google to try and track it down
"> 3) the staff are key to the selection process
I take you haven't actually been in a store recently"
Depends on the store - there are still people who are knowledgeable about their wares and are happy to provide assistance.
"> 4) buying across internet is otherwise impracticable - DIY materials (pots of paint, length of 2 by 4)
DIY and builders merchants do online sales and they deliver"
Yes, but delivery costs can be prohibitive for larger/heavier/small-volume items. F'instance, BnQ charges £5 for orders under £50, and won't deliver bulk-material orders (e.g. bricks, wood) under £100.
And you have to be at home to accept the delivery, unless you have very accepting neighbours!
"> 5) browsing is fun (allegedly) - garden centres etc
I refute that allegation"
Fun is relative... sometimes it's just nice to get out! This also ties into point 2) which you skipped - it can be useful to gauge physical properties (e.g. the way the fabric feels, the weight, etc) and/or look at potential alternatives.
There's a reason a lot of warehouse retailers have cafes :)
"> 6) I need it now!
... so I order online instead of having to wait until the store opens? (Maybe you closer to that particular store than I do, though.)"
That's a funny definition of "now", especially if you're in the middle of a highly exciting Cunning Plan or dealing with a crisis (leaking pipes, blown PSU, broken window, etc).
Ordering online generally carries at least a one-day delay penalty, barring certain special cases such as the limited selection available on Amazon Prime. Or, y'know, where you order online and then go to collect it... at the store. Which, y'know, has to be open.
Anyhow, BnQ is open until 20:00 weekdays [*], 19:00 Saturdays and 16:00 Sundays, and as nice as it is to play with hammers and powertools at midnight, the neighbours generally don't approve. And much like cockroaches and politicians, you're generally never that far from one - there's half a dozen within 15 miles of where I live, the closest of which is ten minutes walk.
Similar applies to other warehouse stores - and if you're near to a shopping mall, these are often open until 21:00 or 22:00 (e.g Meadowhall/Trafford Centre); many Asda/Tesco supermarkets are 24/7.
And while it can be a tad eerie, it's definitely a nicer, quicker and quieter shopping experience at midnight, even if you do have to dodge around the shelf restockers!
[*} I swear they were open until 22:00 for a while - guess there just wasn't the sales volume!