This is going to break down to two different aspects of the question:
Why different shapes?
Unlike the real world, technological limitations in Star Wars don't dictate almost all aspects of design. Real planes are designed to work based on the ability to generate sufficient lift and thrust to get airborne, which is further complicated by the capabilities of the materials available, the weight the plane will have to lift, and the performance requirements of the aircraft once aloft.
Star Wars technology has moved beyond such things. The Millennium Falcon is capable of atmospheric flight and demonstrates the ability to perform aerobatic manoeuvres that would rival the most agile of model aircraft.
https://youtu.be/8sarFZJl3h0?t=172
Therefore we can determine that Star Wars starfighters aren't constrained by the need to balance weight to lift/thrust, or worry about aeronautical control surfaces, or anything else that real world designers would be required to focus on.
As such, each manufacturer is free to design their vessels on different criteria. Design philosophies are one factor, each manufacturer tends to have a distinctive style for their respective lines of starfighters (and star ships). There's also the required function of that vessel; A-Wings for instance have comparatively large engines, as befits their status as fast interceptors, while X-wings have comparatively massive guns.
Beyond the different manufacturers you have the fact that different species entirely are involved in the designs here. 308
The rounded cockpit of the B-wing seems to favour the design aesthetic Mon Cal prefer (both Legends and Disney Canon share that the B-wing's designer was a Mon Calamari), but the X-wing and Y-wing are both distinctly angular. Whereas all real life planes are designed by humans (or at least, computers designed by humans), and humans are all more alike than we are different - especially as aircraft designers borrow ideas from each other.
This leads us to conclude that real world homogeny is neither necessary or even present in the in-universe design of starfighters in Star Wars.
The second aspect of the question I want to address is:
how that makes that particular ship better than other on its tactical function?
The answer here is... it doesn't follow from what we see on screen that it does.
The Rebellion is generally depicted as scrounging for whatever they can get. Whereas the Empire, with all its resources make ships and starfighters that follow similar lines in vast numbers, the Rebels are shown using whatever they can get. Yes, they may have a few truly exceptional custom works of art amongst their fleet (the Millennium Falcon and the Ghost as examples), but it's shown that they begged, borrowed, and stole most of their starfighters and small star ships.
It's only the B-wings that were designed and built entirely by the Rebellion during the course of the civil war, and these are never shown in number. 963