As you noticed in the descriptions of the companies above, page load speed is a major selling point for many of them. With visitors that expect fast loading and search engines like Google using load speeds as a factor to determine search rankings, it can have a major impact on your site and your business.
Security. WordPress is an incredibly popular content management system, and with the popularity and the fact that it is open source comes some added security risk. While it is possible to address security concerns on your own, these hosts take added precautions when compared to standard hosting.
Hands-free maintenance. While the features of each company vary, several offer automatic WordPress and plugin upgrades. If you (or your client) are not interested in taking the time to do these things manually, or if you would feel more comfortable with someone else taking care of it, this can be a significant factor.
Support. With managed WordPress hosting you’ll get support from people who really know WordPress. With most web hosts you’ll get support from people who kind of know WordPress, but for some issues you’ll be on your own. Many of the companies mentioned in this article will even attempt to support issues with themes and plugins in addition to core WordPress issues.