Journal allows me to choose whether to transfer copyright – why would I do so?

To expand a little on user2768's answer, the text provided by the publisher normally explains the benefits and implications of the different options quite well.

• If you choose to retain your copyright, you are only giving the publisher permission to publish and sell your work, but all rights to your work remain with you. You are in principle free to give other publishers permission to print your work, but there still may or may not be a rider about exclusivity (for a specific time or forever) in the copyright agreement even if you choose to retain the rights to your work.

• If you choose to transfer your copyright, the publisher becomes the owner of this piece of text, similar to if you had written a book through a contract with them. You cannot simply turn around and sell or give away the same work to another party.

The (theoretical) incentive for you to transfer is that if the copyright is with the publisher, the publisher has the legal opportunities and incentives to protect their intellectual property, for instance by taking legal action against plagiarized versions of your manuscript. If you retain your copyright, the publisher basically can't know whether you have given another publisher permission to reprint your work, and they also don't really care since it's your work, not theirs.

However, in practice a publisher is only willing to protect your work to the extent that they suffer actual financial damages. For instance, if an obscure spam open access publisher publishes a plagiarized version of your article, you may be very annoyed but the publisher is unlikely to take legal action (or at least none that goes beyond sending them an unfriendly email), since the actual financial damage for them is very, very limited. In that light I agree with you that for most people retaining their copyright is probably the more natural choice.

given the option to still have the paper published while retaining the copyright, is there any reason to still transfer the copyright?

There are benefits of a publisher holding the copyright, e.g., they can enforce copyright, whereas you likely cannot.

When royalties are involved this is particularly useful. When they aren't, it can still be useful, e.g., the publisher can handle instances of plagiarism. (Perhaps readers can list other benefits below, or write their own answers.)

I suspect plagiarism is rare, so I see little benefit in giving-up copyright, and I'd personally keep the copyright.