The analogy that holds is a phone number and a building. The domain is the number people dial, and it points at wherever you tell it to. The hosting is the premises, containing the actual files that make up your website. Changing premises does not require a new number, and that separation is what makes moving a site straightforward when it is set up correctly.
The mechanism connecting them is a set of records held at your domain registrar, telling the internet where to send traffic for your address. Changing hosting means updating those records, which takes minutes and propagates over a few hours. If the domain is registered somewhere independent, that change is entirely under your control.
Where they are bundled, moving becomes considerably harder than it should be. A host that also holds your domain has no incentive to make leaving easy, and businesses in a dispute have found their address held hostage in a way that affects their email as well as their website. That scenario is uncommon and it is severe enough to justify the trivial inconvenience of two logins.
Email is the part people forget, and it is the reason this matters more than it appears. Your email is also directed by records at the registrar, which means anybody with access to that account can redirect your mail. It also means that hosting problems and email problems are frequently the same problem wearing different clothes, and knowing which service is failing determines who to contact.
Register the domain in your own name or your business name, with your own contact details, and keep the account attached to an email address that is not at that domain. That last point is a circular dependency people create without noticing: if your registrar account is tied to an address at the domain, and something breaks, the recovery email goes somewhere you cannot reach.
Enable whatever transfer lock and renewal protection the registrar offers, and set a calendar reminder for the renewal independently of automatic renewal. Automatic renewal fails when a card expires, and the notification goes to an address nobody checks. A lapsed domain can enter a redemption period with substantial fees, and in the worst case be registered by somebody else.
Understand that hosting quality matters less than most comparisons suggest for a small site, and that the difference between providers is dwarfed by the difference between an optimised and unoptimised page. Choose on whether you can export everything yourself, whether the renewal price matches the introductory one, and whether support responds.
Then know which one to contact when something breaks. The site is down and email works means hosting. Email fails and the site loads means mail configuration. Nothing resolves at all means the domain, and that distinction saves an hour on the phone with the wrong company.
Keep a record of which provider holds what, with the account email and the renewal date for each, because this is exactly the information nobody can find during an outage. A few lines in your password manager alongside the credentials converts a stressful hour into a phone call.
Understand that changing hosting does not require changing your domain, which is the practical consequence of the separation and the reason it is worth maintaining. Moving a site is updating a record rather than a migration of your identity, and businesses that bundled the two frequently believe a host change means starting again.