That's the symple answer. :scruff:
The default RPG maker systems do not keep track of time in your gameworld. They do keep track of "Time played" but that is relative to 'real-world' time.
The games you make with the default system are, essentially, linear in nature. You can make an event dependent on the outcome of another event, but
there are no time-based events. ("event" in this case being something that happens in time.)
By adding a "Time" system of a sort, you can add functionality to your game, such as:
Day/Night: changing the tint of the screen based on in-game time to represent the time of day.
Seasons: changing the tilesets based on in-game time.
Time based events: i.e. a shop that's only open during the day. An Inn that only let's you sleep at night. A jousting contest that happens on a specific day.
Calendar: Place a calendar on your main menu showing what day/month/year it is in game time.
Time based quests: e.g. you have to rescue the princess by a certain date, or something happens.
Growing: plant seeds & they grow into food. Maybe they only grow during the summer growing season.
I'm sure there are many other examples, limited only by your imagination.