A copyright is the right to copy an intellectual property. By default, the copyright belongs to the creator with the requirement that the creator's name is labelled on the intellectual property. Anyone else who wants to use or copy the work must have permission from the copyright holder. The copyright holder can also open the work by changing the right to creative common or give up the right entirely by labelling the work as public domain.
A copyright transfer is transferring a copyright holding to another party. The main author loses the authority so why would anyone want to do this? Generally, for marketing, the author may not have the capability to sell their work. Therefore, they rely on publishers and depends on the contract, the author and publisher splits the profit.
On the academia side, authors needs reputation where they will try to have their work published in top journals, proceedings, or reports. Why not do it themselves? Well, it is a big extra effort building the work's reputation and generally, researchers only wants to focus creating and writing, and don't want to be burden with anything else. Top journals and proceedings provides peer review that controls the quality and polishes works. They have great marketing, many audience, great quality, reputation and trust, wide network, many professionals, etc. If you decide to publish yourself, you need to build everything from a scratch.
After Copyright Transfer
After copyright transfer, you lose right to the work. The copyright is now at the other party and they decide the permissions regarding your work. They can give you full permission but usually they only give you partial permission but you are still the author of the work eventhough you don't hold copyright anymore.
Can you share your work? It depends on the party you give the copyright to. If you don't know, you better ask them. If they publicly state that they don't allow to share, you need to ask them for permission and negotiate.
I will share my works on personal websites and blogs since they allow me too however their definition is vague. If it is a server that I have physical access to then it is strictly clear. However, if it is a server where the author is allowed to upload and delete files without the consent of others (e.g., a blog or the server of a university department, preprint server), on blogger, github, and publish0x I can post and delete as I want, but I don't own the platform and they may revoke my right such as being banned or maybe my understanding is wrong that whenever I post and delete is actually based on those platforms' consent. Please leave a comment if you understand.
In my opinion, I think the message is, if the copyright holder requests to delete my post, I can delete immediately, and that is what matters. So, what happens if I am banned on those platforms? Will my posts be deleted or will they remain? If they will be deleted then I'm confident in posting. Please leave a comment if you know the answer.