There is a coherent theme to each parsha in Chumash, and the parshios begin and end where they do for logical reasons having to do with the particular subject matter covered within the parsha. Identifying an overall theme to a particular parsha is generally more challenging in Sefer Bamidbar.
Parshas Matos covers three broad subjects: (1) nedarim/vows, (2) the war against Midyan and dealing with the captured people and property and (3) the desire of Bnei Reuven and Gad to settle on the east side of the Jordan.
[By the way, thematically it is interesting that the unusual word Matos appears separately, one time each, in all three subjects: 30:2 31:4, and 32:28.]
On a simple level, perhaps the unifying theme is verbal expression and the koach hapeh (power of the mouth).
1. Nedarim/vows obviously relates to verbal expression and includes the famous exhortation of k’chol hayotzey mipiv ya’aseh (‘he should do whatever comes out from his mouth’, meaning do what you say you’re going to do). (30:3)
2. While the war against Midyan would not seem to be related to speech, Rashi points out (31:9) that Klal Yisroel’s strength in war derives from the prayers they utter with their mouths, as distinct from Midyan/Eisav who uses the sword for strength.
3. The tribes of Reuven and Gad wanted to stay on the east of the Jordan River. Moshe strongly objected on the grounds that their staying put would be interpreted by the rest of Klal Yisroel as a lack of confidence in their ability to conquer the Land. Reuven and Gad then assured Moshe that that they would send their men to fight the wars of conquering the Land, and only afterwards would the men return to the east side of the Jordan. Moshe accepted this, but he warned them explicitly ‘v’hayotzeh mipichem ta’asu’ – do what you say you’re going to do (32:24). In addition, both Moshe and Bnei Reuven/Gad refer to the east side of the Jordan where they then stood as ‘poh’/here. 32:6 and 32:16. The word poh/here and peh/mouth have the same letters in Hebrew. [Perhaps this is because through speech the peh/mouth gives definitiveness and presence to our intangible and ever changing thoughts. Once spoken by the peh it becomes poh, meaning it has arrived here and is now real.]
Gal Einai, Copyright © 2006 by Gedalia Litke and Torah.org