I'd imagine that it's useful to be able to share a data context between a route-handler, for example, and it's rendered template and the embedded client side "scripting" that may be quite extensive these days.

("scripting" in quotes because of how rust embedded in the html templates can be so much more than logic-less templates- a bit like a server side template, the dom, and clientside javascript all in one- and I'm just guessing, but because it's not dynamic, but compiled, nonetheless safe even though logic is in there...)