Pre studying art and now no longer taking any exams, instead now only being practically or essay assessed, I would say throughout my A Levels and GCSEs I never once took any mock seriously. They were simply too early and weren't prepared very well. I spent my time focusing on revising what I'm struggling on, rather than quickly revising things I already knew just for a meaningless mock. All my mock grades were not a true reflection of what I actually achieved in the end. I would say stick to class tests, but mock exams are just pointless. I can see their advantages but they simply didn't work for me, they're definitely not essential or necessary though at all.