LLMs dont have the same usecase as encoder only models. Lets assume you have around million keywords and you want to find the most similar to a keyword that the user input.

In pre-processing you would have calculated the vector encoding of all the million keywords before hand and now with the keyword the user input, you calculate the vector and then find the most similar vectors

LLM is used by end user, encoders are used by devs in app to search/retrieve text.