You don't have to buy your own domain name, but there are benefits to having one.
Domain names give you a web address which looks better than having a long url. Try telling someone to type in the url above over the phone and it's a bit of a mouthful.
Portability. The website you have created is hosted by e-voice, but if you decided to move to another platform or provider, you would not be able to keep the e-voice url. This can be a problem if you have leaflets and are listed on website directories as you will need to update anything that has your website on it. Having a name that you can take with you is a great solution as it never changes.
Your domain name is also your online identity and you can choose what that name should be for example
wlfg.org.uk or wlfg.uk or westlancashirefootpahgroup.org.uk or anything in between.
The cost is anywhere between about £10 and £30 a year depending on which domain extension and registrar you use.
Having been a website designer for over 20 years I fully endorse the comments by MHayes. A domain is not that expensive but if chosen well will improve your chance of being found on searches and will be much easier to memorise and to use on promotional literature, emails etc.
If (heaven forfend) E-Voice were to cease operating, you would not be stuck up a creek with no paddle.