A lot goes into making a great light:
- LED power/lumens
- beam pattern
- colour temp
- battery capacity
- housing/wiring quality/robustness
- ease of use on bike [adjusting light levels]
- mounting system
- cost
- weight [for helmet lights]
- post-sales service
- won't burn my house down when charging
At the moment I am dealing with older batteries that don't deliver enough runtime. That's frustrating and a bit dangerous as I've had them drop to "survival mode" on me when I wasn't expecting it. So if I am buying something new I want to ensure I won't be starting right up against my desired ride duration since things will only degrade from there and a quality light is going to be on my bike/helmet for 5-10+ years if the current lights I am using are any indication. A more powerful light not only offers the brighter illumination when needed, but allows you to run it at lower levels and extend the battery life a lot.
When I bought my current lights ~10 years ago they seemed like "overkill", but all these years later they are still performing well enough to stay competitive with what passes for a decent MTB bar light today despite lots of tech improvements. Hence buying a new battery and keeping them going.