why did Mr.Piggott always put a candle in the window

Placing the candle in the window is a custom that Mr. Peggotty always keeps. He places the candle in the window to light Emily's way and alert her to her uncle's presence. 

'Theer!'said Mr. Peggotty, cheerily.'Theer we are, Missis Gummidge!' Mrs. Gummidge slightly groaned. 'Lighted up, accordin' to custom! You're a wonderin' what that's fur, sir! Well, it's fur our little Em'ly. You see, the path ain't over light or cheerful arter dark; and when I'm here at the hour as she's a comin' home, I puts the light in the winder. That, you see,' said Mr. Peggotty, bending over me with great glee, 'meets two objects. She says, says Em'ly, "Theer's home!" she says. And likewise, says Em'ly, "My uncle's theer!" Fur if I ain't theer, I never have no light showed.'