There are a lot of (multi)polygons in OSM that are broken in one way or another. And we have to fix them. While some of the broken ones appear on the map just fine, some don’t appear and some mess up the map. And some of those that appear fine on the main OSM map will not show up on other maps where different software is used.

There is too much OSM data! Of course we always want more, more, more data. But when all you need is the data for your city or country, handling 34 GByte of (densly compressed) OSM data for the whole planet can be overwhelming. So one of the most often needed tools for working with OSM data is something that will cut out the region you are interested in from the huge planet file.