When Raith Sienar developed the Twin Ion Engine, he was breaking ground in starfighter story as never seen before. Until then, the galaxy had never seen so compact a starfighter design, not even stripped down Jedi starfighter or Sith starfighters of old. The design had one huge disadvantage, though: heat dissipation. But even with radiators fitted, starfighters so equipped were more compact, less complex than any other design.
By the time Lord Vader had make his own contributions to the design, the technology was proven and refined enough to require half radiator area, efficiently turning more energy to thrust and less into waste heat. That was the case with TIE/sa SFS P-s4, Vader's own TIE/X1 and even some retrofitted army TIE/ln.
But Sienar didn't limit to improve engine efficiency. When it was tasked with developing a newer starfighter, Sienar proposed a new way to dissipate excess heat besides using radiators: blasters. By using excess heat to partially power weapons, less energy had to be devoted to those, powering engines instead. Initially dismissed as dangerous and ridiculous idea, as it required the pilots to discharge weapons regularly just to simply avoid overheating, Palpatine simply nodded and gave Raith Sienar the necessary approval, to imperial tacticians dismay.
I am barely hinting here: by doing this, you are making pilots less hesitant and even prone to fire on anything. Civilians, not knowing the technical explanation, would perceive this conduct as reckless, easy trigger pilots, dangerous and to be feared. If some pilot accidentally or intentionally blows bystanders, all of that is fine with the Dark Side.
The Prototype TIE Interceptor had a third the radiator area of the original TIE, with no less than eight blasters around it, less powerful than the intended ball-cockpit lasers every TIE had sported before. Required into service before the new engines were compact enough to allow lasers to fit under the cockpit, a pre-production run used four underpowered lasers instead of blasters on wings that still proved formidable in battle. Formal introduction came when engine, reactor, sensors and laser packing matured enough to fit on lower third of ball cockpit. But by then imperial tacticians were happy enough just improving wing lasers and reserving cockpit space for other equipment, a modular bay that eventually, with even tighter packaging of engine and reactor, allowed small enough hyperdrives, shield generators or diverse weaponry options. Even Recon and fire control modules were field developed when no TIE/fc or TIE/rc were available and so TIE/ln was the last generation of TIE that required purposefully built variants for the task.New Republic operative Kyle Katarn wrote:Your generic TIE grunt is just plain suicidal. And the TIE defender jockey is bloodthirsty. But the TIE interceptor pilot, he's suicidal and bloodthirsty. When you see a squad of those maniacs flying your way, you'd better hope your hyperdrive is operational.
Every one of TIE Interceptor derived prototypes sported such a modular bay, with most of such advancements being retrofitted to the ever improving, rugged, modular, TIE Interceptor subsequent batches.
This post is intended to somewhat explain why there are lots of TIE/Ln and TIE/Sa variants when there are but a handful of Interceptor variants. At the same time, it is purposed to address upgraded versions seen under Thrawn, as well as Royal Guard continued use of the type when more advanced prototypes were available (although, as prototypes limited to some campaigns and sectors, nor TIE/Av, TIE/D or MIS saw widespread use until NIF implemented those in large enough numbers).
P.S.: Further clarification is required about the use of blasters. Up to the Interceptor, using volatile tibanna gas powered laser near radiators would be thought as suicidal. Sienar rather than dismissing the idea outright, thought of using it as additional coolant but warning the pilot when sensing temperature of the gas was reaching dangerous levels, to discharge it in a controlled way. It was highly risky for the pilot, more so for the inexperienced one, but it effectively took heat away of the fighter, although at inefficient use of tibanna gas.