You don’t say what kind of car you own, but surprisingly, it matters.
My Honda Civic has a pretty small battery, and a pretty small engine. When my battery starts to go, I notice my engine cranking a little slower than it does normally, and I know it’s time to get the battery tested (for free at an auto parts store). Sometimes, when the battery has lasted longer than I expected, and it starts to show signs of age, I just replace it instead of getting it tested.
If you drive a similar small car, and you notice things like a slow-cranking starter, and you carry jumper cables, there is no reason to replace a good, but old, battery. I’d keep driving, and get the battery tested again in six months. It won’t cost you anything.
If, on the other hand, you drive something with a big engine and a big battery, you might not notice when your battery is nearing the end of its life, and replacing it proactively might make more sense. I’m cheap though, so I would never replace a good battery. Instead, I’d make sure I have a good set of jumper cables in the car. I also have a lifestyle that wouldn’t suffer if, one day, my car breaks down. I have an understanding boss and a job where nobody will die or suffer if I can’t make it to work. If you have a different lifestyle, and a breakdown would cause significant hardship, replacing a good battery might make sense.