Rav Moshe Chaim Luzzatto opens Mesilas Yesharim with this questionable claim:

הקדמה – אמר המחבר: החיבור הזה לא חברתיו ללמד לבני האדם את אשר לא ידעו, אלא להזכירם את הידוע להם כבר ומפורסם אצלם פירסום גדול. כי לא תמצא ברוב דברי, אלא דברים שרוב בני האדם יודעים אותם ולא מסתפקים בהם כלל, אלא שכפי רוב פרסומם וכנגד מה שאמתתם גלויה לכל, כך ההעלם מהם מצוי מאד והשכחה רבה. על כן אין התועלת הנלקט מזה הספר יוצא מן הקריאה בו פעם אחת, כי כבר אפשר שלא ימצא הקורא בשכלו חדושים אחר קריאתו שלא היו בו לפני קריאתו, אלא מעט. אבל התועלת יוצא מן החזרה עליו וההתמדה. כי יזכרו לו הדברים האלה הנשכחים מבני האדם בטבע, וישים אל לבו חובתו אשר הוא מתעלם ממנה.

Introduction — the author said: : This work, I didn’t author it to teach people what they did not know, but to remind them of what they already know and is famous among them, very well known. For you will only find in most of my words things which most people know, and concerning which they entertain no doubts. But because they are so very well known and their truths revealed to all, so is the hiding from them rampant and forgetfulness prevalent. Therefore, that there benefit will not be gotten from this work is not in a single reading. Because it possible that the reader will not find that he has learned anything new after having read it that he did not know before reading. The benefit comes from the review of it and persistence, whereby he will be kept reminded of those things which people naturally forget, and will take to heart his duty which hi hidden from him.

I say this claim is questionable because in my experience, there is a lot in Mesilas Yesharim which are not ideas everyone knows.

However, the Ramchal is making it clear that his intended purpose is not to teach us new concepts and inform the intellect. Instead, he is providing a tool for keeping an idea in mind until its internalized. So that it undermines the escape mechanisms we use to avoid our duties in life, and keeps these truths in both mind and heart.

I want to compare the Mesilas Yesharim’s self-description with a quote from the Haggadah, the second half of Avadim haYinu:

… וַאֲפִילוּ כֻּלָּנוּ חֲכָמִים, כֻּלָּנוּ נְבוֹנִים, כֻּלָנוּ זְקֵנִים, כֻּלָנוּ יוֹדְעִים אֶת הַתּוֹרָה, מִצְוָה עָלֵינוּ לְסַפֵּר בִּיצִיאַת מִצְרַיִם. וְכָל הַמַּרְבֶּה לְסַפֵּר בִּיצִיאַת מִצְרַיִם הֲרֵי זֶה מְשֻׁבָּח.

… And even if we were all wise, we were all brilliant, we were all learner elders, we all knew the Torah, it would still be a commandment for us to talk about the Exodus from Egypt. Whomever enlarges their discussion of the Exodus from Egypt is praiseworthy.

Notice the similarity — the Hagaddah too self-describes as not being specific to those who need to learn or even better understand the narrative.

The point isn’t to learn the story of the Exodus on the seder night. (At 50, it’s “been there, done that”.) It’s to internalize its lessons and keep them alive in our psyches as we go through daily life.