Consistency is one of the key signs of a website designed by a professional web designer and not somebody who is learning the trade. Ensuring your website design is consistent all over the various types of pages and forms gives the website a unified feel and pulls it all together, improving the user experience and increasing conversion rates. Consistency is not only using the same colour scheme all over the website, or a similar set of fonts. It also includes making sure the website behaviour is consistent.
Making sure all the different page templates on a website have a coherent look and feel makes the entire site look more professional. Your visitors won't wonder why suddenly things have changed and even if they are still on the same website. Leftover pages from previous website versions look outdated and out of place, and may lead the visitor to believe that the information there is incorrect or out of date.
The best way of ensuring aesthetic consistency on your web design is keeping it simple. Creating a colour scheme with no more than 5 different colours (and that's pushing it) and two font families should be enough for more websites. You can then play with intensity and size to create as much variation as you want, but keep in mind that once your visitors are used to something (for example, H1 tags being a certain colour and size) they will see anything different as confusing, or even worse, as a mistake that looks unprofessional.
For many people, each new website they visit involves a learning curve. Once your users have learnt how something on your site works, they can use it much quicker and their use experience improves. However, if your website rules keep changing arbitrarily or lack of consistency makes it look like there's no logic to it, they may decide that it's too much work, and just leave. People on the Internet want familiarity and order, and a website that fails to have internal logic isn't appealing enough.
To improve your site's consistency in terms of behaviour try to make sure that the same key elements of web design always represent the same, and keep buttons and menus looking similar both in terms of order, colour and size. This way users will be able to identify them at a glance, and won't need to think. The less they need to think about how to access the information on your website, the more time they can spend enjoying it or even buying your products or services.
Even if you are not a web designer by trade, learning to identify the characteristics of a good web design can help you better communicate your freelancers what you want. You can read more about web design for WordPress templates here.