VER is where the importing country induces exporting countries to reduce their exports of a commodity voluntarily. It is a sort of self-imposed quota by the exporting country.
The effects are almost exactly the same as quotas, except that the quota rent now belongs to the exporters rather than importing license holders.
Given this disadvantage, why would some countries use it? The reasons are political: VER gives the appearance of continued support for the principle of free trade, avoiding imposing tariffs or quotas. For example, the United States asked some countries to take VER for automobiles and steel.