This necessitated having all of these emulators and sometimes even older or modified versions of them, along with a great many plugins, to be able to play most of the N64 library with the least amount of issues possible - though admittedly a good amount of games (particularly the most popular ones) were playable with just the best few of them. Each had advantages and disadvantages, and some games worked well in one only to not work in another, even when using the same plugin configuration. Of these, the big three emulators were Project64, 1964 and Mupen64. Historically, the majority of N64 emulators all shared the same plugin spec (known as the zilmar spec, after the creator of Project64, the first emulator to use it), and could therefore all use the same plugins, meaning you could take a plugin DLL file, use it on one emulator, then take that DLL and use it on another, and it would also work there.
As for the terms HLE and LLE, which will occur with frequency throughout this page, and the difference between them, it is recommended to read this page on High/Low level emulation beforehand. To understand the current plugin situation, and why there are several competing emulators that all appear to use the same plugins but said plugins are not compatible across emulators, a bit of history is in order.