Why we need the TOF Zoom! There goes a particle! How fast was it going? In the case of an electron at typical cosmic ray energies (billions of electron volts), it was probably going very close to the speed of light. A proton or nucleus (He, Be, Fe, Si ...) at the same momentum would be going a lot slower. The Time-Of-Flight (TOF) system is a speed trap for particles; it sees each particle when it enters the magnet barrel, and again when it leaves. The TOF measures the time elapsed between these two events, and this tells you the speed. We expect to get about 2% precision; that is, we can tell 96% from 99% of the speed of light, but we can't tell 99.5% from 99.8%.
The TOF also tells you whether the particle was going upwards or downwards. Upgoing particles are dangerous; if you see one going through the tracker, and mistake it for a downgoing particle, you will derive the wrong value for the charge. You'll mistake matter for antimatter, which we very much do not want to do!
In addition to these timing functions, the TOF gives an independent measurement of the particle's charge.