Sovereign Chess exists in a complex world of many neutral armies. It is easy to see all of the pieces of the board as potential enemies, and want to get rid of them proactively, before your opponent can take control. Yet, the conflict is with your opponent, not these neutrals--and so, a piece may only be captured if the opponent controls it.
This has many useful strategic elements, as demonstrated in the following scenario:
Figure 19: Potential Checkmate for Black
In Figure 19, White is in trouble, as the king has retreated behind a small Pink wall with pieces he controls. However, Black has moved his Queen into a potential checkmate position, where it can capture the Pink pawn at d2.
Normally, White would try to fortify his position, perhaps by pushing his pawn to d3, where it is better protected by the Pink pawn at c2--but this has problems as well. The Red pieces, controlled by Black, are poised to attack those same Pink pieces, thus adding desperation to White's defense.
White's best response, it seems, comes in Figure 20:
Figure 20: Promoting a Pawn and Creating a Wall
White's promotion of the pawn at i7 to a White queen at i8 achieves two goals. First, it gives White a stronger piece, and immediately threatens the supporting Black bishop. But, more importantly, it makes the Pink pieces invulnerable to capture, as they are no longer being controlled by White. This blocks the potential checkmate, making Black reconsider its method of attack.
One possible move for Black is to move its pawn from h12 to h11, which would respond to control Pink, and would put White in check. However, White could then response by capturing the Pink rook, as those pieces are now being controlled by the other player.
As you can see, controlling other colors is vital to Sovereign Chess--even more so when we see Rule #13...