Some people also want to control when major segment merges happen, and optimizing at a known time helps prevent a major merge at an unknown time (which can be equivalent to an optimize/forceMerge).
The benefits of optimizing (and having fewer segments to search across) will vary depending on the requests. Normal full-text searches will see little benefit (merging a few terms across many segments is not expensive), while other operations that need to deal with many terms, like faceting, may see bigger speedups. -Yonik
