The first time a waterproof bag earns its keep is usually standing at a bus stop watching the sky go the wrong colour, with nothing but a tote full of paper bits that are about to turn to mush. I've had that exact moment, clutching a normal canvas bag to my chest like it was a baby, and promised myself I'd sort this out. I did, eventually, and it changed how I think about rain altogether.
A cheap one will do the job for a few outings and then let you down in a way that feels personal, the zip starts letting water creep in along the teeth, or the coating goes sticky and starts peeling off in little flakes that stick to everything else in your bag. A decent one just keeps going, year after year, through proper downpours and that fine drizzle that somehow gets you wetter than a storm does. Looking after it is simple, you wipe it down rather than throw it in a wash, and you don't leave it balled up damp in a cupboard because that's how the lining starts to smell. If you've never owned one, start with a plain cross body in a seam sealed fabric rather than anything with lots of exposed zips or decorative stitching, because every seam and every zip is a place water can get clever. It won't be the most exciting bag you own but it'll be the one you grab without thinking. These earn their place from autumn right through to early spring, for the school run in the rain, for festivals, for walking the dog when the forecast lied to you again, and for any holiday near water where a normal bag would be a liability. They're less necessary in a heatwave, obviously, though I've still reached for mine on a boat trip in July and been glad of it. What it sits well with is anything practical, a waterproof coat you're not precious about, wellies or decent trainers, the sort of outfit where you've already accepted today isn't a fashion day, it's a getting through the day day. It doesn't look wrong with jeans and a jumper, which is most of what anyone actually wears when it's chucking it down.
A good waterproof bag is one of those things that sounds boring until the day it saves your phone, your lunch and your dignity all at once.