Some days are big travelling days, in which you cover huge distances between your origin and your destination in complex and convoluted ways. I've just had one of those days, getting from the USA to the UK.
Iceland has a small population, most of which is in the capital Reykjavik; the rest of the people are spread out sparsely in a relatively large land. The result of this is that, while there is public transport, it’s minimal, and not a good way to get around, especially if your stay in Iceland is a relatively short one and you need to be flexible. A car is the only practical option for exploring Iceland outside of Reykjavik.