This close-up portrait allows the artist to show more detail in his painting. There is a large area left untouched below the head, suggesting that the piece was never fully finished. If he had intended to just include the bearded face, he surely would not have left all this space below.
There is a dark palette for this painting, very much against the original teachings that he experienced in Venice. The likes of Titian, Giorgione and Bellini would all use bright colour schemes that was perhaps the most famous signature of the Venetian school.
The most famous artists who made use of darker colour schemes would have to be Flemish artist Rembrandt and one of the true Italian masters, Caravaggio. From that point onwards, many artists that followed would also make use of their styles within their own respective oeuvres.