Very few politicians have the time get down and understand the issues enough to make an informed decision, which they have aids and use lobbyists to learn about the subject.
A decision about deciding about subsidiaries for specific crops for instance, lets say that a farmer used to farm wheat, but then realized that he could get more money by farming tobacco, ok, so he switches to tobacco, but the nation still needs a stable supply of wheat, so wheat needs to be subsidized by the government to make it worth it for farmer to farm wheat, most politicians won’t know if there is a need for this or how large it needs to be.
This is where lobbyists come in, they inform politicians about what they believe is needed, show reports and other data, to influence the politician about how to vote and what to argue for. Wheat farmers and baker advocacy groups will argue for high subsidies, tobacco farmers and cigarette companies will argue against it.