Because it's the largest number that can go into both of those numbers evenly with no remainder.
The GCF of 18 and 36 is 18.
The GCF is 18.
The GCF is 18.
The GCF is 18.
It would be 18.
The GCF is 6.
The GCF is 18.
The GCF is 18.
The GCF is 18.
The GCF is 18.
The GCF is 2.
The GCF is 18.