Wooden stairs lose their aesthetic appearance over time, to a degree that depends on the loads and type of wood. The varnish turns yellow, changing the original color of the wood. Dust and dirt are dented into the varnish. Damage marks and scratches appear. 
In the case of wooden steps that are not covered in any way, the center area shows signs of heavy wear. 
First the varnish and then the wood wear off, which can lead to pitting. 
To remove this damage, all stair treads must be refinished and sanded.

Below is an example of how to refinish wooden stairs. 
Renovation goes quickly, and the stairs look like new.

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