Prior to the days of the Internet and easy international money transfer, it was quite difficult to move money internationally. You could mail currency, but that was dangerous, since international mail wasn't especially reliable, but also inconvenient since converting small amounts of currency was difficult and expensive. Conventions and fanzines didn't have credit card accounts, so even if a fan had a credit card, it probably couldn't be used. Checks were a pain since if foreign banks accepted them at all, they usually charged a heft collection fee. Finally, for many during the War and for many years afterwards many countries (including Great Britain) has stringent capital controls which made it very hard t move money internationally.
The solution for this was the agent, who acted in lieu of a Worldcon or a fanzine in a foreign country. A fan could pay an agent for a membership or a subscription to a fanzine in his own currency and using comfortable local methods. The agent in turn would either transfer the money in an occasional large lump or would hold it and use it to offset local expenses.
For example, a US convention with a British agent and a British GoH, might well use some of the funds collected by the British agent to pay some of the GoH's expenses in pounds, this saving two conversions. Or a British fanzine might discover it was cheaper to send all the US copies as a parcel to its agent who would then mail them domestically and use the money they had collected to pay postage.
There were many ingenious workarounds!
Once things like Paypal and easy ecommerce became available, the use of agents dropped off substantially.