It's just about moving a dependency up a level of node_modules if they are compatible. So if for example 2 packages A and B have the same dependency on babel-core, instead of node_modules/A/node_modules/babel-core and node_modules/B/node_modules/babel-core you could just have node_modules/babel-core.
What's the benefit of this? Saving disk space?
Yes, and install speed too since you're writing the files once
Runtime speed also. Requiring only one copy means only one instance is parsed and optimized by the js engine. Which saves time and heats cache.
Yes, but only if you aren't already transpiring minifying code such as for front end frameworks