The main relay powers your ECU and your ignition system, amongst other things. And when they fail, you're stuck. It's a mechanical relay, and after a couple of decades they just get tired.
Typical failure is immediate. The car will be working fine, then it simply won't start. This will strand the car wherever it sits - at a restaurant, at the beach, in your garage. We've even seen them fail mid-drive, which just shut the car down. Once the relays fail, they almost never come back.
Luckily, it's an easy fix. Swap out your relay as preventative maintenance or toss a new one in the glovebox. Yes, they're a bit expensive but they're cheaper than a tow. When you do remove the original, take a good look at the condition of the socket in the fusebox. If it's corroded, we recommend cleaning it to prevent a recurrance.
Mazda part JE16-18-811. Fits all NA and NB Miatas.