Each one is something a hosting business otherwise does by hand, every time.
Recurring invoices on whatever cycle you sell - monthly, annual, biennial - with tax rules, reminders, overdue suspension and the payment gateway attached. This is the reason most people buy it: chasing renewals by hand stops being possible somewhere around the fiftieth customer.
When an order is paid, WHMCS tells the server to create the account and emails the customer their details. With cPanel, Plesk or DirectAdmin behind it, a hosting order completes with nobody touching it. Without this you are creating accounts manually at whatever hour the order arrives.
Registrar modules let a customer search, register, transfer and renew a domain in the same basket as their hosting, and the renewal invoices follow automatically. The alternative is reselling domains out of a separate registrar account and reconciling two systems.
One login where a customer sees their invoices, services, domains and tickets, and one queue where your team answers them. The client area is the part your customers think of as "your website", which is why it is usually branded to match.
The honest half. WHMCS is the default rather than the only answer, and it is not free.
WHMCS is commercial software, licensed per installation and bought before you install it. That is a running cost on top of hosting, and it is the single thing that surprises people who assumed it came with the server. Check the current price with WHMCS directly - it changes, and any figure quoted on a third-party page is a figure that will be out of date.
The official requirement is an environment with PHP and MySQL, which ordinary hosting provides. In practice the things that break an install are the ones nobody checks first: a cron job that never runs, a PHP extension the installer wants, or a loader the licence check needs. This is most of what "managed WHMCS hosting" means.
WHMCS publishes supported versions with end-of-life dates, and an unsupported install stops getting security fixes while still holding every customer card detail and address you have. Staying current is not optional maintenance here.
If you have five customers you invoice once a year, WHMCS is more machinery than the job needs and the licence is a cost against no saving. It earns its keep when the admin would otherwise take real hours - roughly when renewals stop fitting in your head.
Plans and the practical pages behind the explanation above.