For this specific bit of code the answer is that the programmer wants to set the pins to a known state/voltage before setting them as outputs. But in general, the TRIS bits do more than just set a pin to input or output mode.
The TRIS mnemonic stands for tri-state. When a pin is tristated the resistance of that pin goes so high that they're effectively disconnected from the circuit.
If you've ever looked at busses you'd have seen tri-state circuits. They're the poor man's way of connecting multiple outputs together without using a multiplexer or a switch. All you need is to make sure that there is only one output on the bus that is not in tri-state mode at any point in time.
There is also a "safe" method of implementing busses by not allowing any device to output any voltage other than zero volts. The values on the bus gets VCC voltage via one or several pull-up resistors. Lots of serial busses do this. If I remember correctly CAN bus does this. Contention is never a problem with this scheme. You just need to take care of collisions which can be done in software.
Therefore in a lot of code I've worked with (and indeed in a lot of my own code) the PORT bits are hardcoded to
0 and the pins are driven by setting the TRIS bits to
0. So you will sometimes see code in production where the outputs are controlled via the TRIS bits rather than the PORT bits.