The simplest way to understand a manuscript assessment is from the term itself. A manuscript assessment is an assessment in the form of an editorial letter or a report.
As the term suggests, it is an in-depth edit that helps develop the work into what the author wants it to become. It is also called substantive editing and is sometimes replaceable with line editing.
A book proposal is a blueprint of your work that shows the publisher what your idea entails. Imagine going for an interview with requisite skills but poor communication skills. Will you get hired?