Occupation, colonization, and oppression require resources: monetary, physical, public relaions (propaganda), and diplomatic. Money comes from direct aid (e.g. several billions per year in US aid), sale of products (even to the occupied people), tourism, and from foreign companies investing in the country engaged in occupation. Targeting all sources of money by political action and economic boycotts is thus a very effective way of resistance. In the case of controlled economies, the occupation tries to limit options for buying other products. Using creative ways to find alternative products is thus required in some cases. Boycotts are important tools and not just for products but for academic and cultural aspects. No laws can force someone to buy things that support oppression or to interact with entities and individuals taht are part of the system of oppression. Divestments require removinfg financial support that prop the oppressive system. Sanctions are tools to prevent dealings with oppressive governments or other entities. These three elements: Boycotts, Divestments, and Sanctions (BDS) work in tandem.
BDS started with the inception of Zionism just like in South Africa it startted with the inception of Apartheid. However, in both cases it gained momentum decades later. In the case of Palestine, the BDS movement accelerated in the past 10 years especially after the Palestine Civil Society Call to Action.