Templates provide a framework for your website — a coherent, attractive canvas for you to paint the content of your site onto. They’re how you can have a site that looks good without having to hire a designer. Templates dictate color scheme, what your homepage header and menu bar look like, and the content width on your site, so it’s essential to pick the right one.
I personally don’t think site builders will ever replace web designers/developers completely. Most site builders are targeted at small businesses and could never meet the demands required for larger businesses with all their complex requirements. I think Shopify plus is the only product trying to take on the larger CMS platforms right now (e.g. Magenta, Demandware) in the eCommerce space