Why do railroad tracks have wires?

Many reasons. The main one is to conduct electricity from one section of rail to some other place.

It can be train detection

 A train in the "block" completes a circuit to operate rail crossing signals and other indicators. @ Old Man Dirt

It can be part of the grounding return electricity for overhead or third rail trains.

They can be attached to things like seismic monitors, automatic brake applications,

switch detection (to show if correctly set)

Most common is "block" detection of a train to activate crossing lights and drop arms and change the signal lights for trains following allowing only one train in a block.

It's part of the system that senses where the trains are and signals them so they don't hit each other.

That looks cut.       

Some system detect a train by the wheels & axles shorting across the two rails.

They wire links make sure there is electrical continuity across the rail joints within each section, bypassing rusty steel joints that may not conduct well.

And with electric trains, the tracks are the power return from the overhead power cable. Link wires across bolted joints again guarantee a good electrical connection even if the mechanical one is not too good.

Sensors to detect faults.

Signaling devices. A train in the "block" completes a circuit to operate rail crossing signals and other indicators.