Why do some people believe that high tariffs promote prosperity?
High tariffs are a means to reduce cross-border trade. They are taxes levied on imports and can be used by the government to restrict the entry of products made in foreign countries. Tariffs can be so high as to stop an import altogether or can impose the addition of a sufficiently high tariff that makes these products more expensive and less attractive than products manufactured by local industries.
The policy of using high tariffs to protect American industries was used extensively for a long time. The link below gives a brief outline of their use as a means to allow fledgling American companies a chance to grow large enough to be able to compete with more efficient companies in Europe.
The argument for using high tariffs as a means to increase prosperity has always been that of increased employment rates, better wages for workers, and a compulsion for customers to buy locally manufactured products. Though this is not an ideal way to achieve the required results, it does increase prosperity for small intervals of time.