The requirement to measure the performance of internal audit is defined by two fundamental considerations: firstly and most importantly the need to demonstrate to the organisation’s that we work in, that we deliver services that are of value to them; secondly the Standards say we should.
Internal audit services cost money. Organisations take a decision to invest in internal audit; sometimes the decision is optional sometimes it is a mandatory requirement in the sector in which the organisation sits e.g. banking. Internal audit needs to be able to demonstrate that the services provided, or activity we undertake, is of value to the organisation.
Value can be expressed in many ways, but for internal audit it is perhaps convenient to think of it in the terms expressed in the Mission Statement contained in the IPPF: To enhance and protect organisational value by providing risk-based and objective assurance, advice and insight.
This article from IIA UK provides you with more insights.