I can't imagine anyone's answer being the same to this question, lol.
For me, it was a desire that I had but waited awhile. I was 28 when my first child was born, and 4 years before my second. So obviously I'm not someone who rushed into it. I've always loved kids; I did a lot of babysitting as a teenager, taking care of my nephews, etc.
Why did I have kids? I've always wanted kids, it was just a matter of when. I wanted to have kids when I was content with having done the things I felt like doing for myself, when I was ready to put myself aside a bit for someone else.
I remember as a teenager I used to say I loved kids - loved them while they were good and loved to give them back when they weren't. The best question to ask yourself if you love the *idea* of a child or the reality, lol. You can always get the best of both worlds by doting on your niece/nephew and getting all the love and cuddles and then giving them back when they are tired of it.
I'll end this way: Children are very rewarding, but never confuse that with easy or even always very lovable. I love my kids dearly, but I am convinced that God made all babies and children of all species so cute and adorable as a survival feature so they could stay alive.