OH, you mean lord Guan, the second oath brother to Lui Bei(Xuande), and Zhang Fei (Yide) in the three knigdoms Era.
The Chinese considers lord Guan as a symbol of power and war. As to the the long beard man is posted in the left of the temples?, well no idea. I think that they believe that he is very important to their culture.
It is very sad to know that this lord was killed by Wu soldiers under lord Lu Meng orders. But myths said that he came back again after his execution to enter Lu Meng's body and kills him.
In the three kingdoms era, Lord Guan Yu is considered to be the second best warrior in the knogdoms after lord Lu Bu; A fierce general that his might could defeat a whole army.
In our days, some martial arts includes lord Guan Yu's weapon in training, I mean the long crescent blade.