Why not just one helmet?

Even the best helmets are not able to protect against every blow. For maximum protection they have to be optimized for the type of blow you expect. Some sports have sharp impacts against pavement, which has zero give. The energy spike is very sharp and short. Other sports are played on turf or something with a little bit of give. That makes an enormous difference in the helmet you need.

Some sports have unique problems. Equestrian all season helmets have to withstand the sharp edge of a horse's hoof, white-water helmets have to deal with "bucketing" in very fast water flows and drain well, hockey helmets need facial protection, football helmets are designed for thousands of blows per season, auto racing helmets must be fire-resistant. The list goes on, and you can probably add an example from personal experience.

In some sports, crashes and other risks are not frequent and the helmet can be discarded when a big impact occurs. Others involve constant falling and many other smaller impacts. For these sports a one-use helmet would be a nuisance, and probably would not be replaced when it should be. So some compromises are normally made in protection to make the helmet multi-impact.