Thumb Sucker, - Yeah!
Sucking fingers is the 'little blowjob'. In fact, many men and women see finger sucking as a kind of smaller version of a blowjob. Women like it when they get their lover going by licking their fingers. Women are also attracted by the erotic tension that gradually builds up as the fingers are inserted into the mouth.
For many women, there is also the romantic aspect. If a woman really likes a man, then she enjoys pampering him according to all the rules of the art, including the use of her lips and tongue.
One reason why men like it when women lick their fingers is certainly obvious to many. For many men, sucking their fingers awakens associations with French intercourse. It often seems to them as if what the woman is doing is something like giving a blowjob.
It also turns many men on when a woman opens her mouth and you can shove whatever you want in there. A woman can signal to a man that she will take anything he holds out to her in her mouth, which can further increase the man's sexual desire.