Albert Barnes Commentary 1 Samuel 25:28

"Forgive, I pray thee, the trespass of thy handmaid: for Jehovah will certainly make my lord a sure house, because my lord fighteth the battles of Jehovah; and evil shall not be found in thee all thy days." — 1 Samuel 25:28 (ASV)

For the Lord will make ... a sure house — Compare 1 Samuel 2:35, 2 Samuel 7:16, and 1 Kings 11:38. Abigail's firm conviction regarding David's kingdom is comparable to Rahab's conviction that God had given Canaan to the Israelites (Joshua 2:9–13). Both women testified to God’s revelation and their own faith. This is undoubtedly the reason Abigail’s speech is recorded.