Measurable objectives are necessary for the teacher, even if they may sometimes be reassessed, because it is so easy to lose sight of one's goals when students experience difficulty or misbehave. It is tempting to simply get to the end of the school day, and lose sight of the purpose of the assignment. Measureable objectives prevent the 'dumbing down' of students' education.

Measurable objectives also ensure that teachers do not become overly wedded to one teaching approach or style. For example, if the objective of the lesson is for students to be able to read a newspaper article independently and research the issues for a report on a current events topic, allowing students to select the subject (even if it is sports or media-related) still fits the objective of teaching students how to...

However, while some measurable objectives may be framed in a numerically-oriented fashion, such as achieving a particular score (being able to identify 100% of the states and state capitals on a geography test, for example), usually objectives relating to student comprehension and skills sets are included along with quantitative benchmarks.
Reference

Landau, Valerie. (2001). Topic 4: Developing goals and objective. Round World Media.