Personally, I try to force myself to put a portfolio together every year of stuff that I’m proud of. It can be a collection of short stories and poems or maybe, if I was mostly working on a long piece, a ten-page segment from that.
Now, this is really just for personal use for documentation of growth and stuff, but it really can be nice to have a lot of work to look back on and think “I actually did that really well”. It’s also a handy thing to have around for when someone is curious about your work. You can just say “Hey, actually, I have a portfolio lying around”. That way you don’t have to look through all of your writing, deciding what you like best because you already know what you like and you already know what’s really good.
Portfolios can be also really helpful if you’re thinking about writing freelance. They usually ask for writing samples and you’ll have a whole portfolio that you can super easily flip through and choose from.