This is the first time I hear about this, and I had observed it maybe once or twice, without understanding what was going on.
By law (Ley de tránsito 2148), you can request a bus stop at every corner (instead of at the planned stops) between 10PM and 6AM or at any time of the day in case of heavy rain.
This law dates back to 1968 but most people (and drivers) ignore it. However, you can report if it the driver refuses to obey.
In CABA there is also Ley 5276 from 2015 that adds that this is not affecting Metrobus and stops along Av. 9 de Julio, and between Lima -Bernardo de Irigoyen and Cerrito --Carlos Pellegrini. You can read the short Ley 5276 here.
¿Puedo subir o bajar del colectivo en un lugar distinto al de la parada?
Sólo podés subir o bajar en un lugar distinto al de la parada entre las 22 hs y las 6 de la mañana o cuando llueve.
Fuera de esos casos, tenés que subir o bajar en la parada.