I had a Bench backpack for years, but that was because I used to cycle to work, so needed to carry clothes to change into when I got there, a lunch box, and a few other random bits like a puncture repair kit, bike tools, pens, etc. but that was pre-laptop/cell phone days.
I still only have a backpack (a Nike that my daughter bought me when the Bench finally fell apart after about 10 years) but at the moment it has my work tablet, my Macbook, sat nav, web cam, notepad for recording journey details for mileage and parking claims, but then I have to put stuff in it when I arrive at the site I'm working at, e.g. LAN tester, LED torch, a couple of CAT 5 cables, a screwdriver set, portable HDD with various apps and tech guides on it, a small paintbrush for cleaning the dust out of server vents, reading glasses, bluetooth headset charger case, security screwdriver bits, iPod and something for lunch.
I have a second bag with documentation that I might need, that I keep in the van just in case I need it, but that's a
cheap expanding case from Argos that I've had for years and has been surprisingly durable for the price.
I wear work trousers with plenty of pockets, so my work and own phones are always in there, as is my wallet and van keys, and some change for the rare occasions when I can't pay for parking with a contactless card or by phone. When I spent 9 weeks in Scotland at the end of last year, I only had to use cash once, to pay £1 for parking at East Kilbride shopping centre that where they had cash-only parking.
When I'm not at work I don't carry a bag around with me.