HeyWhatsApp is a WhatsApp MOD that has been developed from one of the most widely-used of them all: YOWhatsApp. So here we find functions from that app, but it also has its own features which are all about customizing the look of the user interface.
More esthetic customizing for WhatsApp
Usually, anyone that decides to download and use a WhatsApp MOD is looking for a more personalized user experience and getting functions and features which the app's official version, for whatever reason, doesn't include. This usually involves more control of privacy (vis-a-vis other users, not the actual service) or customizing aspects related to esthetics and design.
This version offers us the same as YOWA, but it also offers a more advanced tool for customizing the colors and design of the main interface and of the conversations that we have with our contacts.
Now, this involves a certain amount of risk. WhatsApp does not authorize the use of these MODs at all, so downloading HeyWhatsApp or any other MOD can lead to being banned from the service for a certain length of time which can vary. Neither does it guarantee correct functioning, meaning that we often encounter a lack of stability, such as errors when sending files, receiving messages, configuring status...
As a way of being able to use a more personal WhatsApp it looks just fine, but don't forget that it might lead to other issues, such as those mentioned above.