To show the match in 3D, this observer uses the game's own models, textures, animations and sounds. It downloads them from this server as the match needs them and keeps them in your browser's storage. Your next match or visit then reuses them instead of downloading them again.
Over time this observer may store up to 2.8 GB of data on your machine.
You don't need to wait for any of it. The match starts right away, and each model is fetched the first time it comes on screen. It shows first as a small rough version of a few KB, and full detail follows. A lower texture quality in the settings stores less.
Warm up downloads, in the background, everything this match can need: the buildings and units of every civilisation in it, the map's terrain, and the trees, animals and rocks on it. Then nothing has to load when it first appears. This helps on a slow connection, or before the armies meet. It also asks your browser to keep these files rather than clear them when space runs low, and some browsers will ask your permission first.
You can delete the stored files at any time in your browser's site data settings.