I probably need more context to answer this properly.
Lets assume you are deploying your application over https. So it's encrypted. The MX will not be able to see anything inside of those encrypted streams. It still provides benefits in that it can block access from "bad" IP address ranges.
Lets assume you are terminating the SSL session on the WAF (aka SSL offload) and then forwarding it onto a web server behind it (unencrypted). In this case the WAF can see the full unencrypted stream, and can look for threats inside of those packets, such as SQL injection attacks.
In this case, a WAF woud provide a significant improvement on the security posture on the web app being delivered.
If it was me, and I had the choice, I would not be putting a web server on premise. I would put it somewhere like Amazon AWS. In environments like Amazon AWS all the common tools like this are available. And it is usually cheaper than doing it in-house.
https://aws.amazon.com/waf/