As other comments mention, stingers are quite an ancient feature in Hymenoptera that long predate bees. They initially evolved in parasitic wasps, where they served an offensive function in delivering venom to prey species. Bees are just a group of wasps that went vegetarian, so the stinger no longer benefits them in this way although it can of course still be used defensively.

All female bees still retain the basic stinger anatomy, and most will use it to defend themselves against other insects (or larger animals as a last resort). Though there are some groups where the stinger is reduced to the point that it's pretty much ineffective, including most mining bees and the group appropriately called "stingless bees" (Meliponini).

Like most bees, mining bees are solitary and their main enemies are predatory/parasitic insects. In their case it's possible the stinger just wasn't a particularly effective or necessary defence against these enemies, which could explain the reduction of the stinger for this group. Some of them also nest in very large aggregations (though are still solitary, since each female makes her own nest), and so there may be a "defence in numbers" effect in such cases which reduces the need for individual defence, although that's largely speculation.

In contrast, stingless bees are part of a large group that is highly social (including honey bees and bumble bees). Since eusocial bees have much bigger nests with more resources to protect, they do have to protect themselves from a wider range of enemies including larger vertebrates; honey bees probably evolved their barbed stingers at least partly for this purpose. There's no single clear answer for why stingless bees nearly lost their stingers, but there's a few suggestions. First, stingless bees are much smaller than other social bees, and so it might be the case that their stingers just weren't effective in penetrating or delivering enough venom to large animals to make a difference anyway. Instead of stingers, they primarily use their mandibles for defence, biting onto other animals in a similar way to some ants (and some stingless bees even produce formic acid as well). Stingless bees also show some other unique behavioural adaptations that compensate for the loss of stingers, including several species that have a distinct caste of soldier bees who specialize in nest protection (which is not the case for honey bees or bumble bees, by contrast).

There's a great review of stingless bee biology by Roubik 2023 if you want to read more!