Often game balance isn’t just about making sure that certain parts of the same aren’t over powered purely due to number balancing. Sometimes they also involve a human element.
Back early near the early days of World of Warcraft there were two main damage abilities available to Holy Priests:
Smite dealt had a short cast time and then deal a direct amount of damage and no cooldown
Holy Fire had a slightly longer cast time, did direct damage and then applied damage over time. It also had a cool down.
Despite the differences and Holy Fire being the more interesting of the two no one ever used it before as the damage per second of the two was more or less equal. It wasn’t worth including Holy Fire on your toolbar as Smite would do the job with less fuss. Essentially it didn’t serve a function and was redundant.
Rather than just remove it eventually Holy Fire was improved so that it statistically did more damage than Smite. Due to it still having a cool down it now became part of a rotation. Players would use Holy Fire as a priority when it was available and then switch to Smite until Holy Fire was available again.
By doing this something that could have just been removed created an element of skill. Players could be better or worse than each other and could improve their damage output by improving their timings.
As time has progressed and the game has evolved the synergy between the two has changed. Additional talents have been added that cause Smite to sometimes reset the cool down of Holy Fire allowing players with better awareness to improve their damage by using Holy Fire more often.
Sometimes rather than pruning off dead wood its better to cultivate it in to something new.