As the chief legislator, the United States president is responsible for setting the shape of both foreign and domestic policy by recommending legislation, vetoing or encouraging their party to vote in favor of the legislation they want passed. This applies to public policies only. Traditionally, a president is seen as either strong or weak depending on how many of the bills and legislation that they supported got passed.
The president is not allowed to be a member of congress at the same time that he is acting as president. This makes it more challenging to get legislation the president supports through Congress. State of the Union addresses are a helpful tool in gaining support for the items the president wants to pass because it gives him a public platform to present his ideas to Congress and the public at the same time.