With the exception of mobile phone based providers ( ie ee home broadband) which usually cost more , and are more likely ( but not always) to have data usage limits all service providers need a physical line to provide the service.
The "line" for internet access can carry internet and for the time being analogue voice traffic with openreach ( formerly BT and before that post office and before that GPO) lines these are identified by a number which is also the voice line number .
You ned to think of the land line in a literal sense, as a line that goes over or under the ground which in turn carries analogue voice calls and data , if you do not use the analogue voice part, then you still need the line ( that goes underground or over ground ) to provide a data service ( ie internet)
The "line rental" is included in your broadband bill , and this covers the cost of maintaining the line.
Voice services if you want to use them are paid for on either a pay as you go basis, or with a call package ( ie evening and weekends, anytime etc) so if you do not want to use the voice side of things then you dont have to pay for it .
With so called Full fibre, or fibre to the premises (FTTP) the copper lines are replaced with fibre optic lines voice services - which again you need to pay for if you want to use a traditional style house phone , and again if you dont want a traditional style house phone you do not need to pay ( voice services) are digitised into a data service that runs over the internet known as VOIP (Voice Over Internet Protocol ) and some times sold as "digital voice"
If you have a copper based service, even if you do not use analogue voice services i would recommend getting as cheap corded phone to plug should a fault occur, or for emergency use during a power cut ( power cuts can cause mobile phone networks to fail)
