yea it's a fair question - an IDE is often more convenient when you have the code checked-out locally. This becomes a pain when you work in a organization with potentially hundreds of repositories that you need to search across (e.g., a org stores their 100+ microservices in separate repos, and you need to find all places where they make a request to your service).
I use ghorg in tandem with ripgrep to address that problem. The former is for checking out the main branches of all repositories, the latter to perform the actual search.
I cannot run Xcode on Linux, I cannot run Visual Studio on Linux, I might not have an IDE set up for the language that I want to inspect. Many reasons.
Also, some languages practically require arbitrary code execution to make a build, which I'd much prefer to shove into an isolated VM.
Finding examples of how others implement similar logic is my biggest use case for code searching, but since GitHub copied SourceGraph, I don't have much of a need for these self-hosted solutions.