The problem for me is that I have loads of coats. I often wear a navy "raincoat" for work, but when it was cold a couple of weeks ago I wore a long woollen coat, and this week when it was properly raining I wore my Rains waterproof mac with a padded gilet under it. But not at work I wear a more casual rain proof FatFace jacket most of the time, or if it's really cold a padded waterproof, or out hiking I tend to go for a waterproof outer and windproof lightweight jacket under that to give me options.
I also have a couple of Kagool type things for if I go to the village shop but it's not cold but a bit damp. And a medium weight jacket for shorter walks. After badminton I just throw on a hoodie....
The benefit of a handbag is that I don't have to fish around in all these different coats to find the lipbalm, or the handy tissue pack, or my phone battery pack. The all just live in my handbag. So for work, I choose my coat, pick up my laptop bag, pop my phone and keys in my handbag and I'm off. Not having to decide which pocket is right for each item.
In my handbag will be keys (it's a big bunch because I have front and back door, plus car, plus two each for dp and my mum's houses), phone, battery pack, fold up bag, three lip balms, purse, card holder for my season ticket because I have to carry photo id with it, tissues, a small pouch with medication (paracetamol, migraine tablet, my tummy tablets, spare HRT in case I forget it), pen. Sometimes a small water bottle, sometimes a kindle, sometimes a hat (I'm not a fan of umbrellas).
I loathe having to use my debit card on the tube because it means it ends up in some random pocket and I can't remember where it is. I have an Oyster but for some reason it's stopped working.