That global variable is not strictly necessary. In events 1 and 3 we check to see if the enemy has line of site to the player, if it does we tell the enemy to move towards the player at EnemySpeed. Having the EnemySpeed variable means we can just change the EnemySpeed from 3 to 2 to make the enemy move slower, or change it to 4 to make the enemy move quicker.
We could remove EnemySpeed and hard code the speed values in events 1 and 3 instead if we wanted.
Also, calling the variable EnemySpeed is just a name, we could of called it anything, it's just used to set the rate at which we move the Enemy along the X axis.
Double click the EnemySpeed variable, change it's name to anything and C2 will update the events that used EnemySpeed with the new name.