WordPress is easily the most popular blogging platform around right now, but it is not exactly well known for being easy to add forms to. In fact, there many different form generating plug-ins for WordPress, but not all of them are worth the time and effort it takes to deal with them. In fact, some of the free form generating programs come up short in a few critical ways. Gravity Forms is an exception to this general rule, but they are not the only exception to this rule.
The biggest problem with many form generators that can be used with WordPress is that they are simply difficult to use. What makes WordPress so special is that users do not need to have an advanced degree from MIT to understand how everything works. A difficult to use form generator is next to useless for most WordPress users, and thus they should be avoided like the plague. Gravity Forms is exceptionally simple to use, yet it is still very full featured according to reviews.
Functionality is often lacking in free WordPress form generators as well, and that is one area where Gravity Forms and other high-quality paid plug-in modules offer noteworthy benefits. That is not to be taken as a negative against free form generators, but the bottom line is that you get what you pay for when you use free plug-ins at best. At worst, there could be damage done to the underlying foundation and/or database that powers your site. On the other hand, the ability to work with MailChimp is also an important part of what makes Gravity Forms so impressive, but the support options are attractive too. None of these options are available on almost any free form creation module.
It is also worth noting that while products like Gravity Forms work well with just about every theme on the planet, the same cannot honestly be said of all the free form generators. In fact, many of the free theme generators stick out like sore thumbs without a minimal of code tweaking that may not be easy for WordPress users to do. Of course, some WordPress users become CSS/PHP gurus, but they are probably the exception and not the rule.