Tractate Middot addresses one main topic, the measurements of the Second Temple, in general outline and in detail. Incidental to this topic, the tractate also touches upon the order of the service in the Temple.
The purpose of the tractate is to describe the physical background for the numerous halakhot of the various offerings. These descriptions facilitate a greater understanding of the specifics of these halakhot including the procedure and location of every stage of the sacrificial service and the other Temple services.
Having been taught not long after the destruction of the Second Temple, this tractate serves the additional goal of teaching the measurements and descriptions in remembrance of the Temple and as an expression of hope and anticipation for its renewal. Although the basic structures of the First and Second Temples were the same, they differed in several important aspects. The Second Temple was also not constructed in accordance with the blueprint in the book of Yeḥezkel.
Most of the descriptions in this tractate were given by tannaim who lived near the end of the Second Temple period. Several of the Sages cited were present when the Temple stood, and some even served as priests in the Temple. Since their statements are from memory, there are some disagreements, unclear terms and descriptions, and even on a occasion a report that a particular Sage has forgotten the purpose of something in the Temple. Massekhet Middot is also one of the few tractates without a corresponding tractate in the Tosefta, so there is no additional primary source to clarify these questions.