Disk IO can be handy when you have bloated plugin, theme or a page builder, which many WordPress sites have.
Many of them feel uncomfortable creating a subdomain from cPanel. You can’t expect them vetting their plugins for bloatware/poorly written code – it simply is beyond most users technical skillsets. One of the very first points of using WordPress is requiring no technical skill. Expecting a bloated website is a reality.
But don’t expect to having more disk IO at bargain prices. Those servers probably have bigger issues than IO.