When energy is low
On a tired or scattered day, twenty minutes is easier to start than twenty-five or fifty, and starting is most of the battle. It is an honest trade: you give up some depth for a lower barrier. A short block you actually begin beats a long one you keep postponing.
What twenty minutes will not do
It is not enough to get deep into hard problems, where the first ten minutes are often just loading the context. Use it for steady work, or chain a few blocks. The break is five minutes, and the next block is armed when it ends. If you want more room, try the 25-minute or 30-minute timer.