SSRs leak current, so with the SSR "off", there's still a bit of juice going to the element.
More importantly, if you fire your element with the kettle dry, you'll blow it. Most of us like the extra security of a physical disconnect controlled by a switch we can manipulate, to be SURE we only fire the element when we want to. On my setup, I'll have contactors controlling power to both the 120V RIMS and 240V BK element, plus a contactor to control main power to the panel. Feels better having complete, physical control of current flow.