cPanel is used to manage your sites and domains, while Web Host Manager (WHM) manages multiple hosting accounts/multiple cPanels. For example, setting limits on the amount of memory allocated to each account or limiting each account's number of domains.
Web designers and developers often use WHM to offer their clients hosting packages.
Note: Shared hosting plans do not come with WHM. All cloud and dedicated servers have the ability to access WHM with a cPanel license.
Can't I Add Client Domains to My Shared Hosting?
In essence, yes. But it's not ideal. Here are a few important reasons you should consider using a cloud or dedicated server if you plan to offer your own hosting.
If one of your clients has a site with lots of traffic or runs heavy scripts and plugins, it can affect all the sites on your shared hosting account. All the sites set up with your cPanel account will share the resources for your shared hosting account.
Imagine you have five clients with sites on your shared hosting cPanel account. One of them installs a malicious plugin, or their site gets hacked. Now you, along with anyone else you're hosting on that account, are compromised as well.
Time is money.
Additionally, if they need new email accounts, forget their password, or need subdomains added, you'll be the one who has to set it up or reset passwords for them. And clients may not accept that they don't have access to their own cPanel and can do this as needed when they see fit.
Using a single shared hosting cPanel account to offer to host, while possible, is a bad idea all around. It's better to use reseller hosting, a cloud, or a dedicated server. You can then assign them their own cPanel, decide what they can and cannot do, and decide what resources are available to them.
They won't have to run to you every time they need something. Everyone's site will have their own resources assigned to them, providing better speed and performance. And it's much better for security, which is an essential point with the number of security threats roaming today.