It is not any rule or something that you need to provide the domain registration along with the domain hosting. But it is always convinient for the clients to have the domain registered as well as the domain hosted with one company itself.
Some customers already have their own domain names either because they registered it themselves or they are moving in from another place.
Domain names can be purchased quite cheaply from several places which offers domain names only services. It is a nice to have if you are offering web hosting but as the rest said, it is not necessary. It may be convenient for some.
If you do offer, make sure that it is registered in your customer's name and not in your name.
If you are not offering them, you can point them to somewhere that does or you can help them sign up manually if needed.
I see Domain names as just value-added add-ons like web design and web site promotions. Nice to have but not something which forms the core of web hosting services.