Excerpt from watchman docs
With a proliferation of tools that wish to take advantage of filesystem watching at different locations in a filesystem tree, it is possible and likely for those tools to establish multiple overlapping watches.
Most systems have a finite limit on the number of directories that can be watched effectively; when that limit is exceeded the performance and reliability of filesystem watching is degraded, sometimes to the point that it ceases to function.
It is therefore desirable to avoid this situation and consolidate the filesystem watches. Watchman offers the watch-project command to allow clients to opt-in to the watch consolidation behavior described below.