A project portfolio manager would first help in project selection - they would define expected outcomes and business value for projects in their portfolio, facilitate the development of estimates to inform the investment amount, analyze the ROI (ie. is the outcome worth the investment), and then make recommendations about prioritization of projects.
As projects go through execution the portfolio manager would monitor the business case (because estimates and scope are going to change), track and report out on progress and help the team resolve issues or obstacles that come up.
After a project launches a portfolio manager should measure outcomes to see if the business value was delivered.
A project portfolio manager often has a team of project managers that report to them, so leading a team can be part of the role too.
Christopher ’s Answer
The job duties of an investment portfolio manager are to optimize the investments within a client's portfolio, within the guidelines set by the client. Depending on the client and what they specifically are asking for, the PM may be responsible for setting the weightings in various asset classes, eg what percent of the portfolio will be in stocks, what percent of the portfolio will be in bonds, etc. Alternatively, the PM may be asked just to focus on one asset class and then focus on making the best investments within that sector. For example, if a client hires a PM to manage stocks, the PM would try to buy the stocks they think will go up in price the most and sell the ones they think will drop in price. The PM's duty is to try to make the best investments according to the client's wishes.