A miscellaneous rule in Biblical Grammar states that the pronoun for himself when used in a sentence functions adverbially to indicate that the activity of the sentence was done for personal reasons.

A good example of this occurs in Nu10-01:02 And HaShem spoke unto Moses, saying: Make for yourself two trumpets of silver; of beaten work shalt thou make them; and they shall be unto thee for the calling of the congregation, and for causing the camps to set forward. Rashi comments on the personal aspect of making the trumpets by providing three possible methods of personal aspect.
• Make the trumpets for yourself--only you can use them.
• Make the trumpets from your own funds
• The trumpets are used for your assemblies

We now turn to Ex18-27 which concludes the chapter describing how Jethro recognized the superiority of the Jewish God over other gods. The verse states And Moses let his father-in-law depart; and he went for himself into his own land. Rashi explains the personal aspect of Jethro returning home: He went home to transmit his new found belief in the Jewish God to his family and friends. Here Rashi gives a reasonable interpretation of personal aspect that is consistent with the overall chapter contents.

Advanced Rashi: The more scholarly reader will note that there appears something missing in this Rashi interpretation. After all even if for himself connotes personal aspect and even if Rashi's interpretation is consistent with the chapter, still there seems to be a giant leap from personal aspect to a statement that Jethro went to convert his family and friends. Perhaps Rashi gained support for his idea using the reference method. Verse Ju04-11 states Now the Kenite club, who was of the descendants of Hobab the father-in-law of Moses, had separated himself from the Kenites, and pitched his tent near the terebinth in Zaanannim, which is by Kedesh. We are not told what the nature of this Kenite club was; all we know is that they separated from the other Kenites and that the club was founded by Jethro, Moses father-in-law. It seems reasonable that this Kenite club was devoted to discussions and informal teachings and presentations about the Jewish God which Jethro had discovered from his son-in-law, Moses. Such club functions are very common. The focus on a different religion would also explain why the club had to separate from the other Kenites.

Notice how this Rashi is made more plausible by the combination of two Rashi methods-- the grammar method and the reference method. This, the use of two methods, is typical of advanced Rashi.

Acknowledgement: There are many Biblical verses with the words for himself. Rashi consistently interprets them as connoting personal use. After compiling a list of them my brother, the Honorable Neal Hendel of Beer Sheva pointed out that Ex18-27 should be in the list and needed extra explanation. This observation led me to spend considerable time on it till I found the reference in Judges to bolster the Rashi.