Are you asking under what grounds are trade secrets enforced? In the U.S., that would be
18 U.S. Code Section 1832: Theft of Trade Secrets. The USC are passed by Congress so it's applicable in the entire nation. If you're asking what's the point, the idea is that it helps foster fair competition. Suppose a company founder has to dilute their ownership by taking on millions in investment to finally get a breakthrough. You don't want a system that allows an employee to immediately walk away and start a company of their own with that knowledge and completely undermining those who put in the money and effort for R&D.
But, maybe to your point, just because it's illegal doesn't mean it's easy to litigate. Much of the legal system is specifically to avoid litigation. Apropos to this discussion, even if a contract isn't enforceable, it only has to be perceived as having teeth to give it value. It's like when a baby elephant is tied to a stake and it grows to an adult still thinking that stake prevents it from leaving. All it takes is for an employee to think a non-compete has merit to keep them in place.