This is because a portfolio review is a process of evaluating the performance and value of IT investments in relation to the business objectives and strategy. A portfolio review can help to identify the alignment, contribution, and optimization of IT investments, as well as the risks, issues, and opportunities for improvement. A portfolio review can also help to communicate and demonstrate the value of IT to the board and other stakeholders, as well as to support decision-making and prioritization of IT resources.
Some of the sources that support this answer are:
1: This source explains the value of IT governance and how it can help to optimize risk and manage resources to support the organization's mission, goals, and objectives. It also discusses some of the governance enablers, such as principles, processes, and policies, that can help to align IT with the business context.
2: This source provides a research-based methodology to improve IT governance and drive business results. It suggests that conducting a portfolio review is one of the steps to redesign the governance framework and ensure that IT investments are aligned with the business strategy and deliver value.
3: This source defines IT portfolio management as a discipline that enables organizations to manage their IT investments as a collection of projects, programs, and services that contribute to the enterprise's strategic goals. It also describes some of the benefits of IT portfolio management, such as improving alignment, optimizing value, reducing risk, and enhancing transparency.

Discretionary investments are those that are not mandatory or essential for the business, but may provide some benefits or opportunities. The IT strategy committee should evaluate whether the discretionary investment supports the corporate goals and aligns with the business strategy, as this is the most important criterion for IT governance and value creation. The technical feasibility, the business and technical scope, and the alignment with the EA are also important factors, but they are secondary to the strategic alignment and value proposition of the investment. References := ISACA, CGEIT Review Manual, 7th Edition, 2019, p. 92-93; Rethinking traditional technology budgeting processes; Discretionary Investment Management Definition, Benefits & Risks.