Because. Teemo. I would not suggest this build as a top or mid, but supporting your bot lane. You will freeze slow everything and your shrooms will lay the pain.
Flash: Teemo is very squishy, even with the 2 giants belt, so flash is a must have to get away.
Teleport: You can teleport to your shroom if you have one in their jungle, and continue placing more!
For tanky champs I would take last whisper, for more power take rab's cap
First guide, hope it helps someone!