There have only been like two instances ever in my very short life in programming where I have tried writing in a code base that I haven't contributed to from the start. The first time was a sort of open-source volunteer group thing and I still don't understand a single thing that is happening in that code base, though I managed to modify an API call enough to contribute to a ticket.
The second one was just this week where I got to pair with someone who has been working in that code base for about a year. I'm not sure how much I contributed, but if I had to pick a project to work on right now in order to save the world or something, it'd DEFINITELY be that one.
Having a guide in a new world of code is just as helpful as having someone show you around a new city. Sure, you can probably figure it out if you try, but it's also a great way to get totally lost and make bad assumptions based on incomplete data. Plus, in both instances, sometimes your "fresh eyes" and willingness to ask questions can even help your guide get a better grasp on something. Or at least I'd like to think so.
New code bases are scary, and I hope that one day I'll have the skills necessary to just dive in and learn on my own, but in the mean time, if I'm given the opportunity to pair with someone, I'll take it. Heck, I plan on taking it even if I somehow manage to become the absolute best at diving into new projects all on my own.