“Now Abraham gave all that he had to Isaac; but to the sons of his concubines, Abraham gave gifts while he was still living, and sent them away from his son Isaac eastward, to the land of the east.” Gen. 25:5-6
Isaac receives all the inheritance, as he is the son of the promise. Ishmael was already sent away back in Genesis 20 when Ishmael was ‘mocking’ Isaac:
“Therefore [Sarah] said to Abraham, “Drive out this maid and her son, for the son of this maid shall not be an heir with my son Isaac.” The matter distressed Abraham greatly because of his son. But God said to Abraham, “Do not be distressed because of the lad and your maid; whatever Sarah tells you, listen to her, for through Isaac your descendants shall be named.” Gen 21:10-12
And now in similar fashion, the sons of Keturah are also sent away from the presence of Isaac. This was likely done in part to avoid any confusion with the inheritance; no one would be nearby to challenge Isaac after Abraham passes away.
Regarding the term translated as ‘concubines’, more than one source believe this should simply be translated ‘concubine’, singularly. Sarah was the first, thus she was a wife with a marital contract. Hagar was referred to as a wife as well as Keturah, but they do not have marital contracts, as they are ‘wives’ but of second stature, which is why they could be ‘sent away with gifts’ and it was not considered divorce in a legal sense.
The Babylonian Talmud (tractate Sanhedrin 91a) provides an interesting anecdote; that among the gifts given to the children of Keturah by Abraham were the secrets of the dark arts, which according to Rashi was an unholy name to invoke presumably for the practice of sorcery. With what we know from the scriptures themselves and what we’d like to think of Abraham, this may sound implausible; certainly we do not want to think Father Abraham would have anything to do with such things. At the same token we do know that Abraham’s father Terah used to worship other gods (see Joshua 24:2) so perhaps Abraham was exposed to this. As to why Abraham would pass this information on is another matter; surely he could not think doing so would be pleasing to God. Sorcery was not yet condemned in the scriptures, but ultimately is according to the law.