I have seen many videos and why is hugging so important after getting her number ?

It is not. But a hug is good for building comfort.

It releases oxytocin into the HB's bloodstream. Oxytocin is a "feel good" hormone associated with pair bonding, by hugging just after the number exchange it anchors your number with the pleasurable feeling on an unconscious level. After you have broke the hug keep light contact with the girls arms then move one of your hands to point at your eye- the girls gaze will track your finger till you're making eye contact-hold the eye contact for about three seconds and her and your brain will start releasing Fenylethylamine and more oxytocin into both your bloodstreams causing pupil dilation, the second you see the dilation slowly say "Hey" Now the word "hey" is anchored to the pair bond creation and the Fenylethylamine induced excitement.
Break off the eye contact and go your seperate ways. Five minutes later text her; one word "Hey" this reinforces the anchor on the tail end of her hormonal cocktail.

From now on always start your first text of the day to her with the same word.