Moving more is always worth it if you can. If all you can manage is normal walking, then do that. If you can walk briskly, do that too. If you can work out vigorously, do that too.
How much "sub-brisk" walking will help you depends who you are.
For people who are recovering from major surgery or a rough birth, or who have another condition like morbidly obesity, walking might be challenging or even a maximal effort. And remember, anything is better than bed rest! So if for whatever reason you can't do more exercise right now, then walk and be proud.
For an elite endurance athlete, walking will probably have marginal benefits. For an elite superheavyweight powerlifter, even slow walking might be good active recovery or even cardio.
But for most of us, walking at a sub-brisk pace has marginal health effects. It's good for us but not tremendously so. The health benefits we want come from more intense efforts. The easiest among those is walking fast, but you have to actually walk quite fast, and even that isn't what a reasonable person would call the minimum necessary for health.
Walking is good, even if it's not brisk. But we shouldn't overstate its benefits.