Phase II (TDMA) would instantly double the amount of traffic channels available. Phase I (FDMA) only allows for a single transmission to take place per-frequency, whereas TDMA allows for two transmissions to take place by alternating time slots on the same frequency. The control channel and data channels are always FDMA, so TDMA only applies to voice channels on a P25 system.
If the system is running well with the channel resources it already has licensed and operational, then there's not a whole lot of cost to benefit radio of implementing Phase II. In some parts of the country, frequencies are very hard to come by, and so TDMA is a godsend in being able to do more with less.
There are other technical benefits of TDMA, such as the ability for a radio to receive while transmitting. This works because the radio's transmitter is continuously turning on/off in very short bursts (30 ms) while synced to a voice channel time slot, and so it's both transmitting and receiving in rapid succession. That means that even with the PTT held down, the radio can still receive signaling such as emergencies, and can be preempted by higher priority transmissions or a system console. The radio can also be told to stop transmitting by the control channel if the inbound signal is very poor.