I believe the idea is exactly to avoid texting, which is something that has been costly for a lot of people--they would either have to pay for packages with some number of texts or pay per message prices which have been extremely high. Apps like that would help people avoid needing to do that as they would use data, which most people already have and use simply by having a smartphone (which would be needed for those apps) as practically all plans require a data feature to be present for smartphones. And while unlimited messaging has becoming more and more available, there are definitely still many people who are on (often older) plans that don't have that.
If you also factor in being able to message people who don't have phone numbers because they are using a tablet or some other device that can run apps for messaging, and also people messaging internationally (which is still often not covered by unlimited messaging and costs quite a bit), makes the use of these kinds of apps that much more appealing to various people.