The lists of battles and tanks can be filtered for various purposes. You might want to look at all tanks of a specific tier, all battles of one tank, all battles within a certain period of time, etc.
The filtering options appear as context menus for both tanks and battles:
Multiple selections across the filtering menus can be made. Clicking a second time removes the selected element from the filter.
By right-clicking a battle in the grid, you can filter on battles fought with this tank.