Compared to official ranking It gives information about who you faced, rather than how deep you went on tournaments. The more you beat strong opponents the more your ELO is high. It can’t be as precise as when used in chess, but it’s still statistically interesting.

They’re a better indicator of how good someone is versus their ATP ranking. Rank is solely a function of performance in tournaments irrespective of the quality of opponents you play and how well you perform against them. Thus with rank you get people like Ruud who coast to slam finals without playing anyone and end up #2 in the world whereas their Elo is still in the teens and more predictive of how they’d fare head-to-head with another like player