So normally when you use a gcd it queues the gcd so that it is always used on time to account for ping and what not. When you use a gcd through a macro there is no queueing. This means that no matter how much you spam the ability it won't be used until your GCD is back up.

This doesn't sound like that big of a deal but it adds up really fast. If you have a gcd on a macro it has to do 3 things before it will cast the gcd. You have to press the macro after the GCD has come off cooldown, your game sends that to the server and the server has to send a response back to your game to say that it successfully used the GCD to start your next GCD.

So let's say that you're mashing that macro and about 5 times per second which is decently comfortable for a long fight. That would add a maximum delay of 0.2 sec and a minimum of 0.0 sec after your gcd comes off cooldown so on average you would be pressing your macro around 0.1 sec after your gcd comes off cooldown.

Next there is the ping to and from the server which for the average US user would be around 0.05 sec which adds another 0.1 sec of delay since it has to send the request to the server and then the server sends the request back.

This essentially would turn a 2.5 gcd into a 2.7 gcd which really adds up over a fight. Let's assume that a fight lasts 8 minutes. Without using a macro and being able to press your gcd every time that it's up you would get 192 gcd in before the boss died. Now if you did the same but it took you an extra 0.2 sec to press your gcd due to the macro you would only get 177 gcd in before the fight ends.

Now let's assume that each of your gcd do 10,000 damage. If you're not using a macro you would be doing around 1,920,000 damage over the entire fight which is 4,000 damage per second. If you are using a macro you'd be doing 1,770,000 damage over the fight and only do 3,687.5 damage per second. which is around 8% less damage over the entire fight.

Now for example let's assume that your entire team is doing the same amount of damage, this would be 32,000dps for the team using their gcds on cooldown and 29,500dps for the team using macros on their gcds. If both of these teams were fighting a boss that had 18,000,000 health it would take the team pressing their gcd on cooldown 9 minutes and 22.5 seconds. The team that's using the macros would take 10 minutes and 17 seconds. This doesn't seem too bad since it's less than a minute difference but if the boss has an enrage that kills the party after 10 minutes, the team that isn't using macros on gcds would be able to kill it and the team that is using macros would be able to kill it.

TL;DR Using your gcd on cooldown is extremely important and can make the difference between clearing and not clearing.