It’s essentially just another word for “straight”, right? I don’t mind either. Just wondering.
Not really. Cisgender just means you identify as the gender that correlates with the sex you were born with. So if you were born a male, and identify as a man, you are cisgender. You can be a gay cisgender male (or female).
I would say, a big reason for the word is because of hormone therapy and gender reassignment surgery today. 100 years ago, you didn't have the choice to become a woman if you were born male. Now, you can pump your body with estrogen and have gender reassignment surgery to come relatively close to being an actual woman.