What is the purpose of this capacitor in the LIS80225 design circuit of the LAN card connected to the PC?

It may be impedance matching.

I read the data sheet for the end termination of the differential line using RC combination. There is no waste of static power consumption. The termination is to eliminate reflection and ringing.

The virtual ground is not at all involved here. R81, R82 and C74 are obviously connected in common mode to suppress the AC components in the common mode. The more common connection method is to connect C74 to ground. As for why it is connected to the power supply instead of Ground, I think it is related to the type of back-pole isolation transformer. If it is a passive transformer, ground to suppress common mode. If it is an active transformer with a center tap, similar to the center tap of the transformer, it should be connected to the power supply.