You land at Juan Santamaría (SJO) near San José or at Daniel Oduber (LIR) up in Liberia. You are tired, the line for the taxi is forming, and somewhere there may be a Claro desk near baggage claim. The catch is that the desk is not always open, and at peak arrival times the wait can run past 45 minutes. Some people end up driving to City Mall in Alajuela just to buy a SIM card.
A MegaEsim plan skips all of that. You install it before you fly, then switch it on when the plane doors open. By the time you reach the rental car counter you already have data. A 1 GB plan for 7 days is $2.69, which covers maps and messages for a short trip without you ever talking to a sales desk.