There are many benefits of using FIX API. You will have total control over the process of transferring and receiving data without any kind of intermediary or middle-ware software. Contrast this with the MT4 execution chain:
Reasons for a client to start using FIX API
The client is not satisfied with existing financial platforms and interfaces because it is impossible to control all system functionality simultaneously.
Limitations of existing interfaces. For example, many well-known platforms need two or more ticks to fill an order, which is unacceptably slow for high-frequency strategies.
A need for safety and maintaining the anonymity of a client's automated systems.
A desire to cross-connect with the liquidity provider.