God promised Abraham descendants, but years went by and Sarah didn’t get pregnant, Sarah wanted children so she thought that maybe she could help God a little bit by her maid Hagar. Abraham went into Hagar as Sarah suggested and she bears a child to Abraham. Then Hagar treated Sarah with contempt instead of being her maid.
God doesn’t need our help, God wants our obedience and faith, not our struggle, and it only messes up things. Hagar having a child with Abraham brought a lot of trouble.