The SONA is the report of the President on the current situation of the country. This is a practice of the executive to deliver to the people the status of the nation as a whole, i.e. economically, socially, and political. This will help the people to be informed of the achievements of the current sitting President and his/her dynamism to the other branches of the government in implementing his/her platforms for the benefit of the people.
As to how it affects law making.. well the needs of the president is addressed to the legislature since the latter is the one approving the budget of the government. One aspect of law making is drafting the national budget. Without budget, no programs for the people.