The presidency changed during the Progressive Era because of public demand.  The divide was growing between the rich and the poor in this country and business was largely unregulated which led to disastrous speculation.  Food-borne illnesses were common due to unsanitary meatpacking practices.  The presidents after the Civil War and ending with McKinley were quite forgettable in terms of their domestic policies.  One reason for this was that people did not think the federal government should take a major role in the lives of regular people.  Another reason was that after Andrew Johnson's impeachment, the presidency was severely weakened.  After McKinley's assassination, Theodore Roosevelt turned the presidency into what he called the "bully pulpit" for causes that he deemed important.  Roosevelt wanted to preserve the environment because he was an avid outdoorsman, so he created several national parks.  As a New York City police commissioner, he fought against corruption, so he took on corruption in business as well.  He broke up monopolies and after reading The Jungle, he sent federal investigators to regulate the food and drug industry as well as meatpacking plants. Roosevelt's handpicked successor, William Taft, broke up even more trusts but he lacked Roosevelt's forceful personality so he was replaced by Woodrow Wilson in 1912.  Wilson wanted to be known for his domestic policies, so he started to investigate Wall Street speculators and he signed legislation which ended child labor.  WWI brought about the end of the Progressive Era as Americans focused their energy on the European war and after the war Americans were suspicious of anything that looked like a leftist idea.  

The presidency did change to some degree during the Progressive Era.  In general, it became somewhat more powerful with presidents taking the lead more than they had in past times.  The major reason for this was that many people had a desire for change during this time.

After the Civil War, presidents were not very important.  There was a relatively pervasive attitude that held that presidents should not be at the forefront of the government.  That role was reserved more for Congress.  With the coming of the Progressive Era, this started to change.  People saw presidents more as the leaders of the country.

One reason for this was that the country had been fairly evenly divided in the late 1800s.  Neither party really had a strong hold on public opinion.  Therefore, it was not really possible for any president to come into office and strongly push for a given agenda.  This changed in the Progressive Era as well.  Now, there was something of a groundswell of support for reform and a president could come in and act more vigorously.

With people showing a real desire for reform, the president could take a stronger role in government.  It helped that the first progressive president was someone with a strong personality like that of Theodore Roosevelt.  But the real cause of the change was that there was now a real mandate for change that presidents could pursue.