If the product you're working on is a long term project of the company, it's likely you'll leave before it's over in which case there will be a future person who'll have to figure out your code.
In such a case, documentation is essential as otherwise this person would have to contact you after you've moved onto other projects and may have forgotten the intimate details of the code making things tricky.
That's why you have at least two people work on every part of the code, ideally together, but you could easily arrange a hand-off. Solo-developer projects are bad ideas for other reasons, notably that if you get hit by a truck the company is SOL.
