Bullets have pointy shape which make them easily damaged at impact, and so unusable for a second time. Ball Bearings dont have any pointy heads, the force on impact will be distributed over the surface because of round shape - enabling a second time usage.
Obviously this is not for combats - you dont want your enemies to use your bullets back on you. Following are some non-combat situations where ball bearings may be preferred on bullets.
1) In animal hunting, as sports or for food. The animals are not going to fire your missile weapons back at you.
2) Practise Shooting. Instead of your NEW trainees wasting bullets each time they fire on a dummy target, make them use ball bearings. Once you are reasonably sure that the trainees can handle hard military life - both physically and emotionally - you let them use bullets for their shooting practise. This save resources because of reuse of ball bearings, and also because of more efficient production of ball bearings compared to production of bullets.
Come to think of it, even in combat ball bearings will be preferred on bullets, because of their more efficient production, provided either or both of following:
1) Your enemies didnt know in advance that you can use ball bearings instead of bullets, or for whatever reason didnt prepare for it, AND cannot make/acquire guns of same caliber as diameter of your ball bearings (for example your enemy unit is too small to devote resources in constructing furnaces for rifles making/overhauling, and cannot get reinforcments). If you are facing many such small enemy units you can take them out one by one and reuse your ball bearings as you go.
2) Your enemy cannot collect ball bearings fired on it because they passed over its troops' heads to some inaccessible terrain.