Many spells with the ongoing tag give a disadvantage to the caster while they are still active. Additionally, some spell effects may not always be desirable to the caster after some time.
Some explicitly give control over the ending of the spell to the caster, such as the cleric spell Contagion:
Choose a creature you can see. Until you end this spell, the target suffers from a disease of your choice. While this spell is ongoing you take -1 to cast a spell.
But others make no mention of ending control, such as another cleric spell, Darkness:
Choose an area you can see: it’s filled with supernatural darkness and shadow. While this spell is ongoing you take -1 to cast a spell.
Is Darkness permanent until actively removed or is it also maintained at will like Contagion?