It really depends on what exactly will they be doing for you.
If it’s a new site then most decent developers build it on their server first and only once you have approved the demo version they will ask access to your server for the migration. So if you want you can do the migration yourself or you can give them access only for a certain time period.
If they are revamping your site then of course they need the access to clone your site to the staging area and once the revamp is done they will migration it back to your server. So they will need access at the beginning & at the end of the project.
Now when I say access then in most cases developers only need access to Wordpress (/wp-admin). Unless you don’t have nor don’t know how to install WordPress for them. cPanel access is needed only when you don’t know how to use cPanel and the developer has to help you. For example, when you’re migrating your website & domain to a new host and you don’t know how to do it. (Although a decent host would do it for you free of charge)
That being said, if you have asked this from your developer and they haven’t been able to give you an explanation then you might wish to consider someone else. The golden rule here is that you get what you pay for. There are exceptions, but for the most part the less you pay the more likely it is that the developer doesn’t have a secure & optimized process.
If you’re still not sure then tell us what the project requirements are and we can tell you which access is needed.