From my experience/workflow, it's because I typically am needing to switch between reviewing the contents of a file and changing my search up a bit based on what I'm seeing. To be clear, for extremely bulky logs or projects, grep + sed + awk are my go-to since even the best text programs have issues with multi-GiB sized files and I want to heavily manipulate the output, but sometimes that's overkill (and when training employees, the same result can be achieved without the burden of having to learn these programs; eventually they will learn, but it's burdensome and additional stress when they have far more important things to be learning when they start)
When I do searches like that, the tool needs to be there and powerful when I want it, but also immediately out of the way. Notepad++ handles this really well IMO by letting you adjust the transparency of the search and having a lot of power to it, the only blemish being how np++ wants to try to lock the search to the main window (which I'm sure can be disabled, I just never took the time to figure out how). But there is so much you can do with the search that it needs space to show its options and power, and that takes up space from the document which means now I have less on the screen to review. This is annoying even on an "okay" sized monitor, and exacerbated on a laptop monitor.