Think back to days of high school or college - and remember when you had a test in a certain subject. Most likely, you studied quite a bit for that test or exam - and probably did well on it. Now, if you were asked to take one of those same exams today, would you remember anything? - Most likely, not so much... because you haven't been refreshing your brain on that material.
That's what happens with our spiritual knowledge. We all study/have studied God's word before. And we probably know it fairly well. However, like any other material we study, if we don't constantly refresh ourselves and study it over again, we're more likely to forget it and not apply it to our lives.
Peter says in 2 Peter 1:12 "For this reason I will not be negligent to remind you always of these things though you know and are established in the present truth. Yes, I think it is right, as long as I am in this tent, to stir you up by reminding you." So even Peter knew the importance of being spiritually reminded on a regular basis.
Studying God's word on a daily basis and reminding ourselves of things that we probably already know is so important. We become foolish when we think we know God's word "well enough." There's always more to learn and we can always use a reminder.