It about 'trust' and competent issues if there are third parties involved like banks.

Many commercial forms that need a witness dont require a lawyer as one. E.g. Tenancy Agreement. The purpose of a witness is to acknowledge that the person who signs the form is the 'right' person and not someone else when he is called to court.

Having said the above, do not simply sign as witness without seeing the person signing the form even if is your friend.