This verse is part of a larger political scheme orchestrated by Bathsheba, King David's wife and Solomon's mother, and the prophet Nathan. It's a key moment in their effort to secure the throne for Solomon, bypassing David's eldest surviving son, Adonijah. Here's a breakdown:

"Go in to king David..." This sets the scene. Bathsheba is being instructed to approach the aging and likely ailing King David.

"...and tell him, ‘Didn’t you, my lord, king, swear to your servant, saying, “Assuredly Solomon your son shall reign after me, and he shall sit on my throne?”’" This is the core of the appeal. Bathsheba is meant to remind David of a promise he supposedly made to her, guaranteeing Solomon's succession. The key here is whether David actually made this promise. If so, Adonijah's claim is illegitimate. If not, Bathsheba is attempting to manipulate David by claiming that he did.

"Why then does Adonijah reign?’" This is the challenge. Adonijah, feeling entitled to the throne due to his age and position, has already begun acting as king and throwing a coronation feast. Bathsheba, under Nathan's guidance, points out the contradiction: if David promised the throne to Solomon, why is Adonijah already acting as king?

In essence, the verse is a carefully crafted question designed to:

Remind David of his supposed promise.
Question the legitimacy of Adonijah's actions.
Prompt David to publicly confirm Solomon as his successor.
Play on David's loyalty and his desire to uphold his word.

The passage is important because it reveals the power struggles and political maneuvering that often accompany royal successions. It demonstrates how vulnerable an aging and possibly weakening king can be to manipulation, and how key players like Bathsheba and Nathan were willing to use that vulnerability to achieve their desired outcome. Ultimately, this plan will be successful; David will confirm Solomon as his heir, and Adonijah's bid for the throne will be thwarted.