"Do you need help?" is a different question than "How can I help?".
The first opens the door for someone reluctant to ask for help. It gives them permission to ask. To let you know if help is needed or wanted.
The latter conveys a desire to be of service. This question shows commitment. It is not a matter of if you will help but merely how.
Both questions say "I am willing to help," but with a differing levels of engagement.