Can you get a domain for free?
The best thing a website builder can offer is a free domain name. Firstly, it saves a lot of hassle. Your domain is there and ready, without having to worry about porting over name servers or changing A-records - you simply choose and get. Who doesn’t want that?! Secondly, it brings the aspects you need to piece together a website to just one place, making everything easier to manage. Lastly, it’s free. Need I say more! Free domain names are popular with website builders as they are arguably the most important part of a website. It is what decides whether or not impressions will turn into clicks (to an extent) and whether or not a visitor will return to your site (also to an extent) - to have such a vital part of your website locked down and free within the first few interactions of a website builder service is a joy.
Once you have your domain name, you can begin building your website for real. With a domain name, you have a basis from which to build your website. After all, if your domain name is nothing to do with the content / service behind it, you are going to have trouble with returning customers, as it is made far less memorable.
Picking your domain name is extremely important and after it is done it is not reversible. Of course, you can always pay for another domain to be registered, but that is a great way to waste money. If you aren't sure what to pick, leave it for a day or so and get it right the first time. If you want to cancel a domain order early, a lot of the time there is a fee of around $20.