Always been on the fence, keep hoping I want to have kids some day, because I sure don't want to go into it with a negative attitude. I don't want to "miss out" and regret having lived a childless life when I'm 70, but I sure don't want anything about that lifestyle today. My wife seems to be much the same way. We always talk about "some day, if we have kids.." but nearing our 40s, some day doesn't seem to come.
I absolutely believe it must be rewarding in a way I cannot imagine -- just as most 'worthwhile' pursuits that take time and effort and learning and growth are. And precisely because the benefits are non-obvious (I don't really enjoy anything about being around children). I've found raising pets to be far more rewarding than I could have imagined, so I'm sure children must be 100 or 1000x that reward.
But, just like I don't need to take on every challenge the world has to offer (someone else can die on Everest, others can become passionate about their skydiving hobby), I'll pick and choose my challenges.
Having kids gives you a chance in making a more significant impression than just exist and disappear.
I don't think anyone should have children because it helps them come to terms with their own mortality.
It's easier to make an impression on someone if you are nearly constantly around them through their most impressionable years, starting at or near birth.
If you aren't the actual (natural or adoptive) parent, the actual parents will probably get tired of you trying to do this relatively quickly, unless you have an unusual relationship with them.
It's better for the environment (the mountain of plastic and CO2 that having kids causes is astounding) and there are plenty of people in the world that are passionate about having kids.