Convenience.

For a non technical user it is much easier to right click a link (for example a reference in a news article) and choose Split View than arrange two windows (which may involve 2 different monitors).

For a Teams/Zoom screen where you are sharing only one app rather than whole desktop, again split may help.

Also good for mental switching, as you can open a link to the side and either promote to it's own tab/close and carry on with same page rather than have it open separately and break the flow

It's just another tool.