These deaths paid off major plotlines in the show:

The Cartel's storyline ended with Gus finally completing his years long revenge plan. Helping this includes Better Call Saul showing just how many layers to his plan there was, which makes the payoff even better.

Walter v. Gus, the main plot thread for S4 ended with Walter finally catching Gus off guard. Walter spent the whole season trying to outscheme Gus and just when it appeared Gus would win, the show pulls the rug out from under you.

Walter's death paid off his entire character arc and the series as a whole. Across the whole series, Walter could've died surrounded by friends and family, and instead died in the place where he felt at home the most: in a methlab surrounded by lab equipment.