In addition to olfactory and gustatory, fishes have two more chemoreceptor systems, solitary chemoreceptor cells and free nerve endings (Finger TE 1997). Asking why do they have all these chemorecepor systems is 'little bit' difficult in the light of evolution. But we can definitely look at this question by using some comparative studies. Hence, this answer will not give you answer to 'why' but will give you enough information to understand roles of these systems.
Reference provided by you explains much of the mechanism of olfactory and gustatory system. I will highlight the important difference between these mechanisms.
- In olfactory system, signal is directly detected and transmitted to Central Nervous System (CNS) by neurons of cranial nerve I (Liu and Zhang 2019)
- In gustatory system, signal is first detected by specialized gustatory cells and then transmitted to CNS via cranial nerve VII (Purves D et al 2001)
As both of them have similar function, it has been difficult to assign specific function to specific system due to lack of robust behavioral assays (Hara TJ 1994)*
Nonetheless, there are following functions in different kinds of fishes are preferred by one system over another,
- Hara TJ 1994 comment that spatial feeding behavior is supported by olfactory system in contrast to gustatory system which helps in consummatory feeding like turning, biting, snapping and mastication**
- Morais s 2016 suggests, olfactory system is involved in kin-nonkin recognition, prey–predator interactions, and territory or homing recognition because it is less plastic while gustatory system only plays a role in feeding behavior.
- Discrimination of Pheromones is done by olfactory system (Lastein et al 2014)
From reading all these papers, I can 'guess' answer lies in the physiology of these sensory systems. Following conclusion is mine and I am no expert to tell whether this is correct. So just take it with pinch of salt :)
Fish has to survive in the water which is filled with tons of different chemical signals. So it has to distinguish these signals from each other. Many functions might not need very careful separation of signal (e.g. searching for resources) while other might need (e.g. which resources to eat). Some functions might need quick response (e.g. presence of predator) while other can be delayed for short time (e.g. migration). If you give responsibility to one system to perform all these functions simultaneously, it might get confused when multiple important signals arrive. From this I can speculate that this would have put some selection pressure during evolution so that there are multiple chemosensory systems.
From the literature cited above it looks like, Olfactory is less plastic and less sensitive than gustatory and hence involved in functions where you need little sensitivity to distinguish between multiple signal. While gustatory is involved where you need more specific selection of chemical signal.
*I could not get any recent paper directly comparing these two systems in fish. My hope is now we will be in more favorable place to perform some complex behavior experiments. **I did not understand difference between these behaviors conceptually, but you can look it up if you are interested.