There are thousands of themes written for WordPress, so why would you need yet another theme? Because it fits your site if it is customized to your needs. Multi-purpose theme, by definition are not created for the purpose of working the way you need them to work.
They are created to be many things to many people. This can be both good and bad.
Good because it can do a lot of different things and fit many needs. But bad because they often come with a lot of overhead. I mean a LOT of overhead. Code bloat is the term used in the WordPress world for these themes that have a lot of functionality but because of that are ‘heavier’ so they load more slowly and are harder for search engines to read.
Theme lock-in is another problem that can negate many of the benefits of using a great CMS like WordPress.
Theme lock-in means the functionality is packaged into the theme – and in most themes you are locked in. What this means to you is that when you want to change the look of your site by installing a new theme, you will lose much of the functionality in your site. Things will disappear, stop working, or just look really odd.
By using plugins for functionality and themes for the look and feel of the site, you avoid theme lock-in.