WordPress is about speed. Most people try wordpress because you can get a website up and running very fast. It is very true, but after the first use, most people stick with it for more complex stuff too.
When you start a website, you want to get it ready in less time, but to have all the features that you like. If you go with a theme that you buy or download from somewhere, you may end up making a lot of modifications. More, if you don’t know basic web programming you may end up hiring someone to do the modifications for a lot of money.
The alternatives to a basic themes are custom wordpress themes designed for you. You know what you pay for, and it is delivered with all the features you want. No modifications needed afterwards. You may think that is expensive. Well, custom work is not for free, someone gives you his time to make a website look how you want. If we thinks for the beginning to all features you want to be added, then he can design everything around it, and later changes won’t appear. If you hire a programmer to make some modifications he also have to rethink all basic stuff to make it to look exactly how you want.
With a custom wordpress themes, you get whatever you want for the right price. If you download a basic theme and hire someone to do the modifications you are unsure of the outcome, but you have to pay the work.