I'm gonna give you an answer from the bottom of my heart, if that's okay. What makes it worth it is that you get to mold your very own person into exactly who you want them to be. And if you're ready to do that, then it will reflect in the child/ren you raise. Sure they rebel but parenting done right will lead to a lifelong bond and lifelong respect from your child who will always be there for you and will make you proud. Bottom line: the child you raise reflects the parenting you put into that child and to me, seeing a well rounded adult individual that can finally reciprocate all the love you've been pouring into it makes it worth it. Allow me to elaborate:
I say this because my parents sucked at being parents. I know it, they know it, everyone knows it. They weren't ready, they didn't really know what it took to raise children, they were selfish, they hated each other five years into their marriage, they were physically and mentally abusive most of my life etc etc. Not their fault, their parents sucked as well and so the cycle continues. But my boyfriends parents were ready to raise children and I constantly see the mark of their good parenting on my boyfriend. They were intelligent, mature, loving to each other in a very realistic way through the years, always had their children's best interests at heart.
So a few parallels I've noticed between myself and my boyfriend that I attribute to good vs bad parenting:
My SO has no trouble communicating with myself or anyone else and seems to talk about the most difficult of topics with ease while I struggle intensely with communication. (Think days of silence at a time in my home once I got old enough to realize that shutting up put me under the radar) I think this is because I grew up in a home of "because I said so" and "shut the fuck up" while he grew up in a home that valued explaining things and discussing every decision to ensure the child understood it and spending time together as a family talking about their day and what's going on all their lives.
While myself and my SO are both fairly intelligent, my SO is a much better critical thinker and is much more capable of original thought and his logic skills are second to none while mine suck. This I attribute to his parents encouraging him to question his surroundings and question conventionally held beliefs. They loved explaining to him all the silly little questions children ask and encouraged him to do his own research when he asked them about life after death, telling him only the facts about it (aka nobody knows) in a rational way. My parents, on the other hand, did not encourage free thinking or questioning. They only wanted quiet and didn't care that I was a blossoming mind that needed fostering. They quieted me every time I opened my mouth, which I learned to stop doing very quickly. They shoved religion down my throat at a very young age, and I firmly believe this deeply affected my ability to think logically and appreciate scientific evidence.
My SO is WAY more capable of building and maintaining healthy relationships than I am. I think this is because the very first relationships a child makes is with their parents. For my boyfriend, this was wonderful. They encouraged him, loved him unconditionally, patiently opened him up to the world and allowed him to be his own person. In my case, I was not so lucky. My very first relationship taught me basically one thing: people are cruel and need to be avoided at all costs. My parents were both abusive so my very first relationship in life was an unhealthy one. This followed me into childhood and teenage hood, where I suffered an extreme inability to make friends and even speak to people while my SO always had a steady group of loyal friends.
TLDR: I feel like this couldn't be answered in a few sentences, so I'm sorry it ended up being a book. Basically the bottom line here is that parenting done right can lead to a beautiful, capable person who will will return your lifetime labor of love when they mature into an adult while parenting done wrong can ruin a person for life. So that's why it's worth it, but only if it's done right.