The Days Count

The Ramban says (23:15) וספרתם לכם – כמו: ולקחתם לכם שתהא ספירה ולקיחה לכל אחד ואחד, שימנה בפיו ויזכיר חשבונו כאשר קבלו רבותינו. ואין כן: וספר לו , וספרה לה  דזבין, [שהרי אם רצו עומדים בטומאתם,] אלא שלא ישכחוהו.  For sefiras ha'omer,  one must count orally, but a zav and zavah who do not have to become tahor, they don't need to actually count the days.  The Nodah B'Yehuda (YD #2 124 note from the son) asks what's the difference, when one wants to become tahor why don't they need to count, just as they say a beracha on going to the mikvah? 

The Meshech Chachma says תספרו חמשים יום. הענין כי ספירה הוא בכל מקום על שיהא משונה מזולתו במכוון. וזה בזב וזבה וספרה לה שבעת ימים, הוא כי תראה במכוון שיום זה טהור ולא ראתה נדה וכן הוא זיבה, וכן קרי ופליטת ש"ז, והרי ימים אילו נספרין במה שהם טהורים ומכוונים מזולתן, אבל השבע שבתות מה ענין ספירתם מכוון, שיהא נשתנה מזולתו, אם לא דזה הוי ספירה ממש בפה,  In other words, sefirah means to designate the days as unique.  For the counting of a zav/zavah the days are designated by the fact that they lead to taharah.  However, when it comes to the counting of the omer, there is no chalos to the day that should make it designated as part of a count toward anything.  Therefore it is the act of actually counting, of explicitly counting the day that creates a chalos of a counted day.  

In light of this idea we can say this may be the intent of the Ramban.  The Ramban starts by telling us that sefirah must be done by every individual.  That means that the sefirah is not merely to not forget the days for if that was the entire yesod of the count, there would not be a need for every individual to count.  That derasha tells us that there is a mitzvah to create a chalos to the day and therefore, to designate these days one must make a verbal count. 

This may be why there are opinions that one cannot fulfill sefira through שומע כעונה for that only helps when all that's necessary is just words but when the words come to create a chalos of a counted day, for that שומע כעונה does not suffice (Kovetz Migdal Or volume 9 by Rabbi Ezra Schochet.) 

The importance of counting every day, Rabbi Nason writes in Likutay Halachos (Pikadon #4-5) is to teach us that every day is important.  By counting the day, one realizes that the days count.  Every day of a person's life is there to accomplish and grow.  Through the days of coming free from the tumah of Egypt until we receive the Torah, we drill this message into our heads and its imprint lasts throughout the year.