Apex technically means how high a golf ball can go. The maximum height (apex) of the golf ball’s trajectory measured relative to the height where the golf ball was launched from (resting point prior to impact) is the Apex.
The maximum height of the golf ball’s trajectory measured relative to where it was launched is important because this will determine how far and in what direction the ball travels when struck by another object. For example, if a player were aiming for an elevated part with their tee then they would want more loft on top so that shots don't go too far offline (reachable) but still be able to hit something close enough at impact.
The height of a shot will vary considerably depending on the combination of ball speed, launch angle, and spin rate. Variations in either one factor can have an immense effect; for example, if you increase your club's swing speed without changing anything else then shots are much higher because there is more power behind each hit. However it really all comes down to what kind of slope we choose our tee box--the closer this gets towards verticality (ie: putting green), typically means lower heights overall.
For some golfers, it is important to know the height they need in order for their shot. For example, an LPGA Tour player with a 6-iron ball speed of 109 mph and looking for optimal carry angle needs 28 yards while on other occasions PGA TOUR players have higher speeds at 130 MPH which requires greater lofting so that 34-yard distance can be reached when aiming high into the air as well as landing softly onto greenside rough or sand traps rather than rolling towards hazards.
Club design differences can affect your shot distance and golf score. For example, a club's loft is what determines how far the ball will fly when hit with it - but not all woods are created equal! The efficiency of flight and other factors come into play too (elevation).
Your golf clubs are like a puzzle. Each club has its own specific function and design that make it different from the others, but when you put them all together they still work in perfect harmony to help get your game on track for success.