In the link to CANUSB that you provided it says,
the CAN network should also be properly teminated in both ends, so if you connect CANUSB in the end of the network, you must add one 120ohm resistor near the CANUSB and then one 120ohm resistor in the other end of the network.
So the CANUSB device does not already include a terminator and you should include a terminator near there if it is one of the two endpoints of your network.
In practice, the CAN terminators aren't always necessary over short cable runs. If you're using a 2 meter cable, for example, then you may find that the communications work successfully with only one terminator or both terminators at one end of the cable. But you should use terminators at both ends just to remove poor termination from the list of possible problems you'll encounter.
From the schematic, I get the impression that the terminator is on-board your device. That may be OK but it means that your device will always be an endpoint of whatever CAN network it's attached to. And it means that you could have at most two of your devices on a single network. If you intend for your device to be used on a network with multiple other CAN devices then you may not want to embed a terminator in you device. Instead, leave it up to the network installer where to place the terminators on the network.
Best Answer
Because the end of transission line has to be terminated with characteristics impedance of transmission line itself. If the line is not terminated, when the signal travels to the end, it bounces back (reflection) and creates standing wave, similarly like sea wave bounces back when it hits the shore.
Any termination that does not match the characteristics impedance is causing reflections, the worst case is short circuit or open circuit.