Wordpress offers hundred of free themes in their
theme repository.
There is no great reason not to use one but it comes with a few caveats.
1. Branding - often there is the authors branding in the footer giving attribution to him/her or their company. This is not a deal breaker but some people would prefer it not be there.
2. Customization - while a theme can look awesome on someones site and you can't get it to do those things without heavy customization by a WordPress designer or Developer.
Wordsrack supports 2 custom themes that have most features without extra software to achieve the results most businesses require. 1.
Avada Preview (used on our site that you're on, Wordsrack.com) and 2.
3Clicks (Preview) - used on this Wordsrack client site -
Beacon Success
The biggest reason why people use a custom theme is that it requires the author who we/you pay for the theme to provide lifetime updates for an only one time charge (approximately $60). This is important as the landscape of the WordPress software changes from a security perspective daily and as upgraded versions are released it is incumbent on the author to have a theme that works with the most recent WP Version, now at 4.7.5