The point of this law would be to prevent overly idealistic candidates who are too principally and egoistically driven and fail to see the pragmatic realism that is demanded in politics. This is often the case of the non-experienced politicians that have been "shielded" from the realities of work.
How would working outside of politics prevent overly idealistic candidates? Political work is still work. Different types of work face different realities. What "realities" should politicians have to have faced exactly?
Making decisions on behalf of other people is a politicians job. In order to gain enough experience and knowledge in the field in which you are making decisions through personal experience seems like a minimum requirement.
Politicians represent people from all types of jobs. They do not have time to become an expert in every job, it makes more sense for them to spend their time listening to the experts from relevant fields. Doing other work would mean less time listening to experts and workers you are representing.
Experience outside of the political system is likely to add to a better understanding and appreciation of constituents' circumstances and concerns, allowing a politician to better consider and set governmental policies for constituents.
This would only be relevant for the constituents they have worked with and not the 99% they haven't. Workers themselves can pass on knowledge to politicians. They could spend that same time learning from different kinds of workers from different fields, thereby representing a lot more constituents