Good questions, ones that are receiving more philosophical treatment recently. As a parent of three I better have some good answers, eh?
First off, I haven't read either the Stanford Encyclopedia of Philosophy (SEP) entry on Parenthood and Procreation or the book Why Have Children? by Christine Overall (see this article), but you might find those interesting.
So, here are five reasons that I think are good reasons to have children, though of course, they might be outweighed by other reasons (such as worries about overpopulation, or lacking financial resources to raise children well, etc.):
1. Human life has value and humans create valuable and meaningful things (such as art, philosophy, humor, and pleasure), so it is good for humans to continue to exist. So, someone should have kids. It might not be necessary for me or you to have children, but assuming it is better for there to be humans than not (and I think it is), then it is necessary for some people to have children. (Also, a younger generation is required to sustain the economy. You won't get social security unless enough people have kids to pay taxes when you are old.)
2. The relationship between parents and children can be a wonderful relationship that adds meaning and pleasure to one's life (yeah yeah, also frustration and pain), so if you have good reason to think your own life and that of your children will gain meaning, value, pleasure, etc. from that relationship, one should have kids. Children can be fun!
3. Children can provide you with grandchildren, which may provide all the good things described in 2 with less of the frustration and pain. At least that's what all my kids' grandparents say.
4. Children can provide you with a way to continue your existence in certain ways, both biological and psychological. (I'll leave it vague, hoping people will fill in the idea in a better way than I could in a brief response!)
5. Teaching has value and gives meaning to one's life. Passing on good ideas, beliefs, capacities, crafts, trades, traits, etc. is both necessary to sustain them and satisfying in its own right. (This may be a specific part of 1 and 2, but when I said I'd give 5 reasons I was just setting a goal for myself...)
Notice that almost none of the reasons I've given require having biological children rather than adopting. Indeed, it is harder to give reasons for why one should procreate, even if one thinks raising children has value. You may want to check out some of the discussions on this question under the "Children" category on the left bar.
Hope this helps!